It would help to know what RV you have.
My first guess would be the water pump check valve is leaking backwards. If that is the case you can get a check valve and install it between the pump and the fresh water tank. This fix would be cheaper than replacing the pump.
Chuck - Sparta, TN
2012 Jay Flight 22FB, 2 x Honda EU2000i
2013 GMC Yukon XL Denali AWD
EDUCATION is what you get when you read the fine print.....
EXPERIENCE is what you get when you don't.