City Water Tank Bypass Issue

There are only two possible options:

1. Leaky (or not fully closed) fresh water fill valve, if you have such a valve to fill from the city water inlet. Some RVs only have a gravity fill, and this wouldn't apply to them.

2. Leaking check valve in the water pump. This most commonly is due to a bit of dirt or other buildup getting stuck in the diaphragm, and sometimes heals itself with a bit of pump use as the speck of foreign material is washed out. It can also be due to wear, in which case the solution is to either rebuild the pump with a new diaphragm (not too hard or expensive), replace the pump, or install a secondary check valve in the suction line to the pump.

The pump normally prevents water from being pushed through it into the tank but sometimes a piece of debris can stick in it and cause it to leak. Using the pump may have already cured your issue. If not you may have to take apart the pump head and clean it. Worst case you need a pump rebuild kit, new pump or install an extra check valve after the pump output.

Good to know. I just hooked up the hose and so far no sign the tank is taking on water. Maybe a day of using the pump jogged it loose. Thanks and feel more confident now.

FYI there is also a check valve in the city water inlet that prevents water from leaking there when pumping from the tank.

Could be possible that the campground water pressure was so high that it over-powered all and any or your check-valves, but your home water pressure is normal and does not over-power and does not cause a leak.

CG water and electrical are sketchy things...

In general use CG water to fill your tank and use your tank as the water source (or only hook up for your long shower and then unhook).
Same with electrical, CGs can have very dirty electricity that can harm your RV.