I added a dockside connection to mine by teeing in to the pressure line after the water pump. This relies on the check valve inside my pump to prevent water from flowing into the tank.
I am thinking of adding a line from the dockside water around to the other side of the pump, with a manual valve so that I can fill the tank like yours is doing.
There are also automatic valves out there that do the same thing. Yours could have this automatic system, and its malfunctioning. Could have a manual valve that is currently open, or could have a bad check valve in the pump, however, if that was the case, your water pump would run nearly continuously, and would never just pump up and stop.