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Depending on year make and model it could be lots of things. Does yours have a cut out switch for both the Chassis and House? Are they on?
Some models don't charge the Chassis battery when connected to shore power, the newer Winnebago's do.
There may also be a AUX start switch on the dash, this cross connects the House and Chassis to start the engine.
If the Generator would not start then the House batteries are dead, if the engine won't start then the Chassis battery is dead.
As you stated you were able to start after it after leaving it plugged in for a few hours, Did you use the AUX start switch or just normal starting?
Will the Generator start? Have you checked the water level in the batteries? What is the date code on them? Are the hookups to the batteries clean?
OK let us know what you find