Why Does My RV Have Two Grey Water Tanks?

Although RV plumbing is an intimidating subject at the start of the hobby, most designs are actually simple. So, why would an RV have more than one gray tank?

If you are laying in the gravel on a hot day trying to make sense of your RV’s hookups by looking at them, it's time to take a step back and go cool off inside while I help you make sense of the tanks and valves under your rig.

There are a few reasons your RV would have another gray tank installed. If you have a large rig it may be factory fitted with two tanks for long trips, or there could be separate tanks for showers and sinks based on floor plan. Lastly, you could be mistaking your black tank for another gray one.

Next we will go over each of these possibilities in more detail and talk about what kinds of RV’s usually feature what option, as well as some tips on identifying the important parts of your gray tanks. An A-class RV with two gray tanks might have a different system for leveling and flushing out your tanks compared to a C-class with a single gray tank.

When I first became a full time RV goer I confidently took my first trip to the tank dumping station and left without even opening the valve on my overfilled gray tank because I didn’t learn enough about my RV before going. Now, I know the system well enough that I could replace the entire tank with confidence if needed. I’ve also done the research for you on what RVs usually have two gray tanks based on size and year.

‍Reasons RVs Might Have Two Gray Water Tanks

RV Tank Capacity Vs the Overall Size of The Vehicle

While 30 gallons is average, large A-class RVs and 5th Wheels are capable of carrying tanks up to 40 - 60 gallons. Some RVs have multiple tanks installed from the factory to provide the owner with more space to store gray water. One single 20 to 30 gallon gray water tank might last a solo camper or a couple for about one week, but generally it is closer to four days.

If you are in a larger RV (30 Feet or more) there is a good chance that your vehicle is simply outfitted with two gray tanks onboard to last longer so you can spend more time using your RV before having to empty the tanks.

How Fast a Gray Tank Fills

A large group of four to six people can easily fill a standard 30 gallon gray water tank in a couple days, or even one. If you’ve purchased a big rig meant to haul the whole family out on prolonged vacations don’t be surprised to find two jumbo-sized gray tanks lurking below your rig.

Some of the larger rigs out there are outfitted with two 60 gallon capacity gray tanks, accommodating up to one month of time between emptying.  Small or large, your RV might have two tanks because it was economic to build it that way. Waste tanks are large and take up a lot of space, but two smaller ones can be mounted in different areas to save room and improve weight distribution.

Can Smaller RVs Have Two Gray Tanks

Long A-class rvs are not the only ones that might have two gray tanks; Purchasing an aftermarket gray tank is relatively affordable and installation isn’t a huge pain either.

Some owners opt to expand their tank storage by installing another tank rather than buy an external one (discussed below). If you bought a used C-class RV there is a chance that what you’re seeing is a second gray tank that has been installed after the original purchase.

Does RV Gray Tank Capacity Vary By Year

Vintage RV owners like myself might find that a second tank has been added because of the small capacity of the original gray tank. Gray water or “wash water” was not always considered as much of a pollutant as sewage tank water and it was more common to drain the gray tank onto the ground without hooking it up, so compacity was not as important for designers.

Vintage 1980 - 1990 RVs that are smaller than 30 feet  are more likely to have had another tank installed at some point because of their limited capacity than newer rigs in the same size category.

What RVs Can Use External After-Market Gray Tanks

All of them!

If your RV has a large gray tank mounted to the back of the vehicle rather than beneath it then it is probably a portable tank. This is an external gray tank and it does the same thing as installing another tank without the permanence.

These tanks are not directly connected to the RV and don’t usually come with a new RV. You will need to use the hookup hose to empty the main gray tank into one of these external tanks.

Portable tanks are equipped with wheels and can be a good alternative for someone needing more gray water storage that either has no room for a second tank, or someone that does not move their RV often.

Valve Differences for RVs With Two Gray Tanks

If your RV does have two tanks there is a good chance that one is designated as the main tank. This means that one tank will fill with waste water as usual and can then be emptied into the second tank when full. Your main tank may be under the shower, while the other connects to the bathroom and kitchen sinks.

If this is the case take time to familiarize yourself with the drain valve that connects your two gray tanks and make sure to know the difference between it and the main drain valve, black tank drain valve, and both gray tank drain valves all leading out to your sewer connection.

What is the Difference Between Gray and Black Water Tanks?

Can You See Your RV Water Tanks from Outside

If you are visually checking your tanks and you see exactly two, that is normal. One tank should be gray in color. That’s the “gray water tank”. The second tank is almost always black in color. That is your “black water tank”. The Black tank will typically be mounted directed under the area your toilet is in.

Other than A-Class and Super C-Class sized Rvs It is more likely that your RV has a single Gray tank and a single Black tank installed than anything else and it is important to know that you have properly identified your black waste tank and open it only when connected to a proper sewage dump hookup.

What are Black Water Tanks

Black water tanks are where your toilet flushes to and only contain this waste. Typically, these tanks are close to the main sewer valve and have a single shaft attaching them to it. Gray tanks are mounted nearby and connect into the same shaft, but not to the black tank directly.

RVs with only a single Waste Water Tank

If your RV is older than 1974 there is a chance that only one mixed waste tank was installed from the factory. Many RVs this age can or have been fitted with a separate Gray tank if they are still in use. Any RV newer than the mid 70’s is highly unlikely to have only a single tank.