The rooftop a/c units aren't designed to be serviceable like residential systems. If anything other than a fan dies, it's replacement time unless you can do it all yourself. The dashbaord a/c system is a standard automotive-style system, probably an Evans-Tempcon system. Any automotive a/c shop can service it if it needs anything. Typically they develop small leaks and need "freon", or the water valve for the heat/cool control gets stuck and heat overwhelms the cooling.
The generator needs typical small air-cooled engine maintenance, i.e. spark plugs and air & fuel filters. That and regular exercise under load to keep moisture from building up in the wire coils of the stator. See the Onan Generator Handbook for advice.