The RV sewer system is a gravity based holding tank system that stores liquid waste until it can be drained into a sewer hookup. RV's typically have two waste water holding tanks, black and grey. The contents of the toilet flush into the black tank. Sinks and showers drain into the grey tank. Both the black and grey holding tanks have separate outlets that lead to a single sewer outlet outside the RV. Some RVs even have multiple sewer outlets to accommodate extra holding tanks. When the black and grey tanks are full, they need to be emptied. Here's how it works: 1) The sewer hose is connected to the RV sewer outlet on one end and a campground or dump station sewer hookup on the other end. 2) Each tank is dumped separately by opening the tank gate valve. Once the valve is opened, gravity forces the contents of the tank to flow through the sewer hose and into the sewer hookup. The black tank is emptied first, and the grey tank second so that the grey water rinses the sewer hose.