You have a backflow problem from city water into the FW tank, most likely cause by crap in the integrated check valve in the pump.
Check and clean the suction strainer for the pump, then turn off city water and run water from a sink using your pump for a couple of minutes. This usually cleans it out and allows the check valve to seat.
Ted & Melinda
2016 ACE 27.1
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2020 Chevy Colorado Z71 Trail Runner Alternate Toad