Hi,
Most commonly the shore power water connection has a back flow valve. Sometimes they stick. I believe there is an "O" ring that may have aged out.
Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.