There should be a check valve that keeps the water from draining out the city water supply when you are using the onboard water pump. Similarly, there should be a check valve in the piping to keep the city water from entering the water tank. There is a drain on the water tank that drains the tank. There are usually drains near the water heater and near the city water inlet. There is also a drain on the water heater itself so you can drain the water heater by opening the little brass valve and the pop-off valve. Look under the unit for hoses hanging down below the belly pan for locations of the various drain valves.
1978 Argosy 6 Metre Minuet
2006 Toyota Tacoma