I would take the coach someplace and fully charge the batteries from shore power before doing anything else. It might take 24 hours to fully charge the battery. Also check the water level in the batteries. If that is low, the battery might be bad (if you can see the plates that is really bad for the battery, and distilled water must be added before further charging or starting the engine again).
Your on board charger should be in the 45 - 65 amp range and should put out about 30+ amps to the battery, fully charging it in about 2 days. But it should be full enough to start the generator in 24 hours.
If it cranks and will not fire up, then starting the engine can help, because once the fuel system is pressurized, it will help 'push' gas into the generator.
Good luck!
Fred.