People overuse the salesman's switch and it wears out. It should be left on all the time. We are often just too lazy to go back, lift the big battery bay door, and do the correct thing by turning off the house Main. Either the switch at the entry or its solenoid/relay in the battery/big fuse area gets worn out and fails, leaving the symptoms you describe.
Check your coach battery connections for integrity of course. As Steve mentions, the refrigerator needs 12v to run even though we think of it as just alternating current and gas powered. The reason is that the fridge's display, if any, and the board that controls everything on it is 12 volt. It won't start or run on gas or 110v without that 12v control.
-Joel
God Speed, John Glenn