I can only answer one of you questions .
In order to have 120v power to your coach from the generator, the power cord must be plugged into the outlet in the compartment or to shore power.
Your RV was built without an auto transfer switch , to save money during production.
Parts supply , depends on the part . For appliances ? Rubber parts for door seals ? Windows ? New or used parts?
If paneling is water damaged , it should be removed to prevent mould.
If your just changing for color you could go over.