To clarify: you are saying that the 12 volts appears when you plug in to shore power, disappears when you unplug, and reappears when you plug back in. Is it both the house and the chassis 12 volts that disappear (i.e. do the overhead lights and the headlights go out when you unplug)? If I have the symptoms correct so far, it sounds like the batteries are no longer connected to the 12 volt circuit, but the charger still is. This would imply that a heavy duty switch/breaker/fuse near the batteries is off.
If you use a multimeter to check the voltage at the battery terminals, is it near 12 volts? Does it start to rise to 13 or 14 volts when you plug in?
As with any troubleshooting of something that worked and now doesn't, ask the question "What changed between then and now?". Any shop work?
Do you have a fuse or disconnect mounted to the battery terminal or very close to it?