Our motorhome only has one check valve on the city water in line into the main supply line, there is no check valve after or before the water pump. Ours relies on the check valves inside the water pump to stop the city water pump from back-feeding into the fresh water tank, I have installed a gate valve at the bottom of the fresh water tank though so that I can clean out the water pump pre-screen without dumping the fresh water tank or flooding the rv. I would check the line between the tank and the pump, and the pump and the main water line of the motorhome