I'll take a stab at a possible reason. When the generator is running, it could/should/might supply both 120 and 12 volt power. Since the steps work on 12 volt, power is available. When she is connected to shore power, the coach has 120 volt, but 12 volt would be supplied by an inverter/converter that should/could be charging the house batteries. For some reason, the battery power isn't being supplied to the steps. Aside from run down house batteries, a possible cause could be the solenoid that is sticking and needs replacement. They look like little beer cans with copper arms on each side and small connections near the bottom. They are easy to replace and are available from any auto parts store.
Since the steps work on generator power, a blown fuse is unlikely. While ground wires are a typical cause of problems, since it works on generator power, that's also unlikely.
Good luck.