My new-to-me travel trailer just did that. There is a checkvalve in the waterpump which is supposed to work... mine failed. Installed a cut-off valve on the waterline coming in (bandaid solution.) Solved the problem. The fresh water tank swelled so bad, the pump actually made a dent in it.
Upside is I can lift the bed, turn the valve and the freshwater tank fills while the water is connected to the city water inlet. Downside is, the water pump does not work correctly (on/off/on/off) while trying to use pump only. Can only have it turned on while using the tank, then make sure it is turned off.
Not a bad idea to have the cutoff valve, but I do have to spring for a new waterpump down the line.
