The first in a new series of RV Basics, today we tackle what would seem like a simple, easy question: What are RV holding tanks? Read on for some expert insight into those sometimes pesky but always necessary components of every RV.

Your RV’s holding tanks make it possible for you to travel and enjoy your RV without being dependent on outside facilities. Almost every type of RV has three main holding tanks: a freshwater tank, containing potable (suitable for human consumption) water; a sewage wastewater or black water tank, where waste from your RV’s toilet ends up; and a wastewater or gray water tank, that is the final resting point for water from your shower and galley (or kitchen) sink.

All three tanks tend to reside below your RV’s living space and each requires specific types of care and maintenance to ensure that they work properly, don’t give you (too many!) headaches, and that they last the life of your RV.

How big are my RV’s holding tanks?

Sizes of tanks vary according to the type of RV you have, the manufacturer, and even the model. In general, the freshwater tank is about the combined capacity of your black and gray water holding tanks. So, if you black and gray tank can each hold 50 gallons, your freshwater holding tank is probably around 100 gallons in capacity, if not just a bit more.

Motorcoaches and fifth-wheel RVs tend to have bigger holding tanks, with 40, 45, and 50-gallon waste holding tanks being very common. Travel trailers often have tanks ranging from 25 gallons to 40 gallons. Some smaller Class B RVs have tiny tanks; the smallest we ever worked on was just 6 gallons in capacity!

Not all RVs will have black and gray tanks of equal capacity. In some cases, the gray tank will be about half again as big as the black tank. For instance, if the RV’s black tank is 45 gallons in capacity, the gray tank may be 65 to 75 gallons.

How do RV holding tanks work?

Let’s be clear here: Your black and gray water tanks are not a septic system; they’re a holding system. They are designed to hold waste until you are ready to empty the tanks into your RV park’s local waste holding or municipal waste processing system.

Many RVers don’t realize that and they try to treat the tanks — especially the black one — like a septic system. If you’re one of them, you’ve probably been told that you need to add a tank additive — enzymes, chemicals, or other biological agents — to break down solid waste. That’s a huge mistake that we’ll discuss in the next installment of this series on RV Basics.

Basically speaking, waste and water from your toilet, shower or sinks travel down a pipe (sometimes there isn’t much of one under the toilet) into the appropriate holding tank. There, the tank fills up until your sensors indicate they are full or you decide that you need to empty them.

When you’re ready to empty — or “dump” — one or both of your holding tanks, you go outside and pull or open the appropriate tank’s gate valve and whoosh, the tank’s contents travel down any piping and into the articulated waste hose that you’ve connected to your RV on one end and to the sewer dump hole in the ground at your RV park on the other end.

For freshwater tanks, water is extracted from the tank via a small, onboard pump. It’s commonly located near other “hidden” appliances such as your furnace or water heater, or sometimes it’s located under your refrigerator or under the main sink area. Water from the freshwater holding tank circulates through your RV’s plumbing system, providing water to your faucets, shower, and even toilet.

That’s pretty much how holding tanks work.

What are tanks composed of?

Most RV holding tanks are plastic boxes composed of high-density polyethylene plastic. They are manufactured by a provider working with your RV manufacturer, often to their exact specifications, though there are “common” sized tanks and configurations.

RV holding tanks have bottoms that slant down toward a short exit pipe, located either under the tank or toward the front bulkhead. Gravity forced the waste and liquids out when you open the gate valve. Waste and water enter the tops of the tank, either directly from the toilet or sometimes after traveling a short distance from the drain or other area of the RV.

What do holding tanks look like?

For motor coaches, most holding tanks are fairly cubical in shape, though they may be longer as they stretch into the middle of the coach. Wastewater holding tanks in motor coaches tend to empty easier and more completely as the mass of water in waste in them drives down and out any contents when emptied.

Holding tanks in travel trailers and fifth wheels, on the other hand, tend to be much flatter. When emptied, though there may be an initial rush of waste and water, it is nothing like that in a motorcoach. That — combined with more horizontal piping — are also why waste holding tanks in these types of RVs tend to drain so slowly and, oftentimes, incompletely. Many travel trailer and fifth wheel RV owners end up putting a gate valve and cap on the end of their waste pipes to catch the inevitable seepage.

Where are my holding tanks?

Depending on what type of RV you have, your holding tanks can be located in different locations. In almost every case, access to them is on the “street” side (or, for motor coaches, driving side) of the Rv, on the opposite side of the door into your RV.

For most motor coaches, they are usually located about 2/3rd’s of the way back, placed side by side. They also tend to be “higher up” in motor coaches, not located under the RV.

For travel trailers and fifth wheels, they are normally placed about the middle of the RV, though some may be more toward the front while others may be in the back. Think about the layout of your RV’s interior: wherever there is a toilet, sink, tub or shower, there’s likely to be a holding tank nearby. Holding tanks in travel trailers and fifth wheels tend to have their tanks pretty low, under the living and storage space.

Stay tuned for next installment of RV Basics as we discuss how to care for your RV’s holding tanks.

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