The water pump internal check valve isnít closing...this allows city water to flow thru the water pump and back fill your water tank...
With city water off and valves set for using water from tank bump the pump off and on with one faucet open ..
Do this several times..
Then drain fresh water tank and leave drain open and turn city water back on with valves set properly city water.
Often just cycling the water pum0 in and off a few times will clear the valve and let it seat.. if problem still continues then remove internal check valve and clean and reinstall
2007 GMC Classic club cab 4x4 Duramax LBZ
2014 Alpine 3010 RE. 34 foot fifth wheel