There is a valve that chooses city water or tank water depending on which one is pressurized. If, for some reason, is damaged or hung-up may be the source of your problem. Note to myself, next winter, put RV antifreeze in tank and use pump to flush and fill with antifreeze.
Be sure to isolate and drain water heater first.
2009 Toyota Tacoma V6 crew cab 4X4 V6 TRD
2014 Flagstaff T12RB