RV Holding Tanks And Plumbing Diagram

Beginner’s Guide To RV Holding Tanks; What You Need To Know

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RVing is a fantastic way to spend time with loved ones and witness some amazing scenery along the way. Your RV or camper is essentially a home on wheels, and it doesn’t matter what type of RV you have, if it has a kitchen and a bathroom, it has holding tanks.

Your RV holding tanks and plumbing components are two of the most important systems in your rig. Neglecting these can lead to a nasty and inevitable disaster at some point. If you are a new RV owner or just want a better understanding of your tanks, this comprehensive guide will help you.

Let’s jump in…

What Are RV Holding Tanks?

RV holding tanks are containers that are designed to store your freshwater and waste water until you can refill or dump at specific locations, like dump stations.

Most RVs and campers are equipped with three main holding tanks that are mounted to the underside of the RV. The fresh water tank, the gray water tank, and the dreaded black water tank.

All three holding tanks are vital to your overall RV plumbing system and each tank serves a different purpose. Here are the three RV holding tanks explained.

RV Holding Tanks And Plumbing Diagram
BASIC DIAGRAM OF RV HOLDING TANKS AND PLUMBING SYSTEM

Fresh Water Holding Tank

As the name implies, the fresh water holding tank stores fresh, or potable, water for the times when you don’t have access to a fresh water hookup. Some instances when you might be required to use your fresh water holding tank are boondocking (dry camping) or staying at various campgrounds that do not have utility hookups.

Filling the fresh water holding tank is a fairly straightforward process. Simply fill the tank with fresh (potable) water that you can find at gas stations, truck stops, campgrounds, and even some dump stations.

Just don’t use the hose that is provided to clean RV sewer hoses, so you don’t risk contaminating your entire fresh water system. The fresh water spigot is typically marked with a sign stating that it is potable or safe for consumption.

Fresh Water Holding Tank Maintenance

Properly maintaining your fresh water tank will ensure you and your family will have clean drinking water when camping off-grid.

If you only use your camper on the weekends, make sure to drain your fresh water tank between uses to prevent any bacteria and mold from growing inside the tank.

To sanitize the fresh water tank, add a diluted solution of bleach and water to the tank. A general rule of thumb is to dilute 1/4 cup of bleach for every 15 gallons of water used.

Once the water has been added to the tank, turn on the water pump and the faucets until you can begin to smell the bleach. Turn off the faucets and let the solution sit for 24 hours in the water lines and the holding tank.

After the 24 hours has passed, drain the fresh water tank, and fill with fresh water. Let the faucets run until you can no longer smell the bleach.

Gray Water Holding Tank

The gray water holding tank is where your shower, sink, and washer (if you have one) water is stored. Some larger RVs and luxury fifth wheel trailers will have a separate gray tank for the kitchen, called a galley tank.

While the water in the gray water holding tank is typically soapy water, it can still be contaminated with certain things like food particles, bug sprays, and cosmetics. Everything you wash off your body and dishes goes down the drain and ends up in this tank.

Gray Water Holding Tank Maintenance

Maintaining your gray water holding tank is much easier than the other two RV holding tanks. The lines that drain into your gray water tank are equipped with “P” traps, much like the plumbing fixtures in a house. These are designed to hold water so the gases and odors from the holding tank don’t creep into your living quarters.

If you begin to smell foul odors in your RV, these “P” traps may be the culprit. Cleaning out the traps with a bleach solution or other treatment made for RV holding tanks will solve this issue.

Black Water Holding Tank

The most dreaded and often feared RV holding tank is the infamous black tank. This is where all the sewage from the toilet goes to be stored until you dump. The RV community is full of fecal fears and poop references when talking about the black tank. See what I just did?

Don’t worry, with the proper equipment and maintenance, dumping the black tank can be completely stress free. I will tell you that no matter how much you know about your camping crew, you will know a lot more about them after your first time dumping the black tank.

Black Water Holding Tank Maintenance

The number one thing you can do for your black tank is thoroughly rinse and flush the tank every time you dump. Leaving solids or toilet paper in the tank will cause serious issues later and could eventually leave you with a clogged drain.

There are some simple steps you can take to prevent a nasty, stinky black tank, and avoid clogging at the same time. In addition, keeping your black tank clean will help the built-in sensors read more accurately. (most of the time)

Always make sure there is plenty of water in the toilet before flushing and always keep your black tank drain valve closed. As far as toilet paper in the tank, some veteran RVers say to only use RV or septic safe toilet paper because it breaks down easier than the traditional paper you get at the store.

We have always used the cheaper, Dollar Store branded toilet paper and have never had an issue. A good test you can perform before using your chosen toilet paper in your RV is to place a few sheets in a bowl or cup of water.

After a few hours have passed, cover, and shake the bowl or cup. If the toilet paper is shredded or broken down, you should be fine. If not, I would consider using a different type, or brand.

As far as treating the black tank, we use Happy Campers RV Holding Tank Treatment. It helps keep the tank clean and deodorizes it in the process. But there are tons of RV holding tank treatments on the market to choose from.

How Big Are RV Holding Tanks?

Knowing the average RV holding tank sizes is an important piece of information to have. Afterall, if you are looking to buy an RV with the sole intention of dry camping, you might regret your decision if you later find out that it has small tanks.

When it comes to holding tanks, the size of each will be dependent on the type of RV. For instance, a luxurious fifth wheel trailer will have much larger holding tanks than say, a pop-up camper with a bathroom.

Every RV is different, and each has its own unique features.

I have included a chart below that lists the average holding tank sizes of the most popular RV types.


RV TYPEFRESH WATER HOLDING TANKGRAY WATER HOLDING TANKBLACK WATER HOLDING TANK
CLASS A70 to 100 gallons40 to 65 gallons30 to 50 gallons
CLASS B16 to 40 gallons8 to 35 gallons10 to 26 gallons
CLASS C35 to 60 gallons30 to 90 gallons25 to 40 gallons
FIFTH WHEEL50 to 90 gallons50 to 90 gallons35 to 85 gallons
TRAVEL TRAILER35 to 60 gallons50 to 90 gallons25 to 40 gallons

To find the holding tank sizes of a particular RV, check the manufacturers website or do a quick Google search by make, model, and floorplan.

If it is an RV you already own, check the owner’s manual or inside a cabinet. You may find a label or sticker inside one of the cabinet doors that lists the holding tank sizes.

Can I Leave My Black And Gray Tank Valves Open?

Leaving an RVs holding tank valves open when at a full-hookup site is one of the most controversial topics in the RV community. You can read 5 different articles or 5 different online forum discussions on the subject and get 5 different points of view. Most, if not all, agree that the black holding tank valve should always remain closed until the tank is at least 2/3 full. Why?

If you leave your black tank flush valve open, what happens when you flush the toilet is the water will drain. The solids will not. I’m sure you can imagine what happens with the left behind solids.

By keeping the black tank valve closed, your black water holding tank will fill with solids, AND water. So, when you dump your RV holding tanks, the solids will get flushed out of the tank with the water.

Now on to the gray tank. There are a lot of different opinions regarding the gray tank flush valve. As for us, we leave ours closed until we are ready to dump. The main reason is to give us a way to flush our sewer hose after dumping the black tank (more on dumping waste tanks below). The other reason is similar to the black tank.

Although the contents of the gray tank are much cleaner than the black tank, it’s not just water in there. Food particles, hair, toothpaste, dirt, and any other solid you wash off of yourself or your dishes will make their way into the gray tank.

And much like the black tank, will settle in the bottom of the tank, and not get flushed out with the water. Over a period of time, these solids will build up causing unnecessary issues down the road.

I would suggest leaving the black and gray holding tank valves closed, even if you at a full-hookup site. Just my two cents.

How Often Should I Empty My RV Waste Tanks?

Certain factors will determine how often you should empty your RV holding tanks. The main factors being the size of your family and the size of your tanks.

If you are traveling with a large number of people, you may have to dump your tanks every couple of days. If it’s just the two of you, you may need to dump your waste tanks less frequently.

The size of your RV holding tanks will also play a role in determining the frequency you need to dump. If your RV holding tanks are small, you will be hitting the dump station more than if your tanks are large.

There are, however, steps you can take to save precious space in your RV holding tanks and lengthen the time between dumps.

  • Consider using the campground showers and bathhouses
  • Take “military” type showers. Instead of running the shower constantly, turn the water off while lathering up.
  • Put a tub in the kitchen sink for washing dishes and dump the used water outside. This also works for shaving, brushing your teeth, etc.

A good rule to follow is dump your waste tanks when they are 2/3 to 3/4 full. Most RVs are equipped with a holding tank level indicator inside the coach that will show you how full they are. However, the sensors relaying this information aren’t always accurate.

RV Holding Tank Level Indicator
Typical RV Holding Tanks Level Indicator

Toilet paper and other particles often get stuck to these sensors causing them to read inaccurate levels. Even newer RVs tend to have issues with tank level sensors. Proper RV holding tank maintenance and cleaning can help alleviate some of these issues, but not always.

How To Empty RV Waste Tanks Step By Step

Now that you have a firm understanding of what RV holding tanks are, and what they are used for, it’s time to jump into what many RVers see as the worst part of the RV experience. Dumping the waste tanks.

Some RVs and campers will have varying systems for dumping the waste tanks, but the overall process is the same. In addition, the process will be the same if you are at a full-hookup site, a dump station, or if you are dumping into a portable waste tote, or blue boy.

Step 1: Remember The 2/3 Rule

The first step to dumping your RV waste tanks is to wait until the tanks are at least 2/3 full as we mention earlier in this article.

Step 2: Level The RV

The old adage rings true, $**T rolls downhill. If the dump station is unlevel to the point where you won’t be able to dump, take the few extra minutes it takes to make sure your rig is level enough for things to flow freely. This is only an issue if it slopes away from the waste tank dump valves.

Step 3: Locate The Waste Valves

The most common location for RV waste valves is near the rear of the coach around the utility hookups. Some motorhomes and fifth wheel trailers may have the valves located inside a compartment. If you are unsure about your specific model, check your owner’s manual or the manufacturer’s website.

Typical RV Waste Tank Valves
RV Waste Tank Valves

Step 4: Get The Gloves

Always keep a box of disposal rubber gloves, soap, and hand sanitizer in a convenient location for when you are dumping your RV holding tanks. You might also opt for a pair of reusable heavy-duty gloves if you prefer.

Step 5: Make Sure You Have A Quality Sewer Hose

When dealing with the contents that are coming out of your RV holding tanks, the last thing you want is a major spill. Cheaper sewer hoses are made with thinner walls and are prone to damage that can easily lead to a sewer hose blowout. For more information on affordable, quality sewer hoses and accessories, check out our RV Sewer Hose Buyers Guide.

Step 6: Secure Both Ends Of The Sewer Hose

Connect one end of the sewer hose to the waste tank connector on your RV and securely connect the other end in the sewer pipe at the campsite, dump station, or portable waste tank. MAKE SURE ALL CONNECTIONS ARE SECURE before moving on to the next step.

Step 7: Always Dump The Black Tank First

The reason behind dumping the black tank first is because when you dump the gray water holding tank, the soapy water will help rinse out the sewer hose.

Once the black tank has finished dumping, connect a non-potable water hose to the black tank rinse, if equipped, and thoroughly rinse the tank. A clear elbow attachment will help you see when the contents from the tank are running clear.

If your RV doesn’t have a black tank rinse, this clear elbow serves as a tank rinser as well. Other options are to drag a hose inside the camper with a wand attachment to rinse the tank from the inside or pour a 5-gallon bucket of water down the toilet.

RV Black Tank Flush Connection
Black Tank Flush Connection

CLOSE THE BLACK TANK VALVE. Before moving on to the next step, make sure the black tank valve is securely closed.

Step 8: Dump The Gray Water Tank

The gray water tank isn’t as bad as the black tank and the process is the same. Open the gray water tank valve until the holding tank is empty, then close the valve.

Step 9: Clean The Sewer Hose

While the gray water tank has already rinsed most of the foulness out of the sewer hose, you will still want to rinse it out with a non-potable water hose before storing it in the RV. Just remember to rinse it while it’s still connected to the sewer, so nothing spills on the ground.

Step 10: Enjoy The Rest Of Your Trip

After everything is cleaned, rinsed, and stored, the next step is getting back to your trip.

How To Find RV Dump Stations

Most RV campgrounds and RV resorts will either provide full hook-up sites or will have a dump station on the premises. However, this isn’t always the case.

RV Dump Station Road Sign
Dump Station Road Sign

If you find yourself in an RV park or campground that doesn’t have a dump station, or if you prefer boondocking, you will likely have to find a dump station to dump your RV holding tanks.

Finding a nearby dump station is simple and effortless. You can either do a simple Google search and follow the results, or you can visit certain websites that cater to RVers looking for a place to dump. Some of the most popular sites are:

There are also many apps you can install on your smartphone that will provide this information. For a complete list of apps that every RVer should use, check out our article on the subject here.

In Conclusion

By this point, we hope you have a better understanding of RV holding tanks and how important they are to the overall RV experience. Keeping your holding tanks clean and well-maintained is the key to a successful adventure…every time.

What are your thoughts on RV holding tanks? Drop us a comment below and let us know. We would love to hear from you.

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