Of course that's no longer the case, mostly anyway, but it helps to explains why the sanitation system on most motor homes, travel trailers and fifth wheels consists of the toilet which drains into the RV's black water tank and the bath tub/shower, wash basin and kitchen sinks which drain into a separate gray water tank. Each tank has it's own valve but they usually dump through the same sewer connection. There are exceptions to this... sometimes bath water will drain into the black water tank.

Nowadays it is generally not acceptable to dump gray water under any circumstances so a few RVs are being built with one combination gray/black tank. The advantage is mostly to the builder because there is less plumbing to do do. But the combination tanks are necessarily larger and it is hard to find a place for them so most RVs still have two tanks.

RV Gray Water Tank

Since only sink and bath water collects in the RV gray water tank it needs less attention than the RV black water tank.

Most people use so much water whenever the RV is hooked up for a few days, they find it convenient to leave the gray water tank valve open.

If you use the RV for more than a week with the gray water valve open then it's a good idea to close the valve, add holding tank chemical and allow the tank to fill. This helps flush out any buildup of stuff from the kitchen sink and keeps the tank fresh. It is quite possible for a gray water tank to get very stinky unless it's flushed periodically.

RV Black Water Tank

The RV black water tank needs more attention primarily to control odor but also to keep solid waste and toilet paper from collecting and plugging the drain line and valve.

The black water drain valve should be left closed until the tank is 2/3rds or more full. This helps avoid the solids building up right under the toilet and assists flushing everything out. If the tank is not 2/3rds full when it is time to break camp, simply add water through toilet.

It's a good idea to close the gray water valve the night before breaking camp. Then when it's time to unhook, drain the black water tank first then the gray-water tank thus flushing the black-water completely through and 'rinsing' the sewer hose. The procedure works but I don't always remember to close the valve the night before.

I use a rinse wand which attaches to my utility hose and goes down through the toilet into the black water tank. It really works fast at flushing and rinsing the tank. With all the freshwater going into the tank eventually the sewer hose is flushed clean also.

I don't rinse the black water tank every time I dump but an occasional rinse does help control odor especially during the hot summer months.

Please don't take the time to flush your holding tanks when there are people behind you waiting for their turn at the dump station. Flush your tank when you are at a campsite with hookups or if you're sure no one will be needing the dump station.

After the black water tank is drained and flushed close the valve and add enough water to cover the bottom of the tank and then add the tank chemical.

It's my opinion that RV holding tanks are just that, holding tanks. They are not mini septic systems. I can't imagine any serious bacterial or enzyme action happening in the two to four days that most RVers 'hold' their black waste. If that were the case we wouldn't need 3" sewer hoses or macerator pumps.

Again, it's just my opinion, but the only reason for using chemicals in the black tank is for odor control. If you're using chemicals for any other reason you're just kidding yourself. I know it is a controversial stand but someone had to say the emperor has no cloths.

You may also want to read: RV Holding Tank Chemicals - the Basics