You probably have separate batteries for the house stuff and the engine stuff. (Maybe not but most MHs do)
Look for a button on the dash near the driver that says auxiliary start or battery boost or something like that. This allows you to connect your house battery bank to the engine battery. This will help start the engine.
The buzzing has me wondering that maybe you have something that is an error-do not start thing. Maybe the emergency brake has to be on, all slides have to be in or a leveling jack isn't all the way up. If I turn my key on with the jacks down, I get a constant ding. Maybe you have a steps down alarm or something.
Hopefully someone will be along soon with the same make, model and year to give you more specifics.
Good luck.