Low Water Use
Unlike household toilets, RV toilets work with only a very small amount of water. Most use about two quarts of water per flush and have a water-saving rinse setting as well. Water does not stay in the bowl on a regular basis. For solids, the bowl is filled and then flushed, while for liquids the bowl can be rinsed using only a small amount of water.
RV toilets, when flushed, empty into a holding tank located on the underside of the vehicle. Most RVs have three holding tanks, one for fresh water, one for "gray" water and one for "black" water. Black water is human waste, or the result of flushing the toilet. Special chemicals are used in the black water tank to eliminate odors and speed the decomposition process. The waste, or black water, remains in the holding tank until it is flushed.
Waste from RV toilets collected in the black water tank is emptied by connecting a hose from the tank to a dump station. Appropriate dump stations are usually located at RV parks and some campsites. Once the hose is connected and put in place, a valve is opened that allows all the waste to drain through the hose into the dump station. RV toilet tanks are usually emptied when they are mostly full to make sure there is enough water pressure to push all the solids through to the dump station.