I've owned Class C and A, two major items in both vehicles were air leaks and window tinting / insulation.
The best time to find air leaks is when its cold and windy, I just found a very large leak in my Mirada, just in front of the gas pedal right at the firewall, probably 2 x 6 inch hole that lead right into the engine compartment. I used a can of spray foam to plug it. (This explains why my feet were always cold when we were traveling in the winter)
Similar air leaks in my class A, doors, window handles, vehicle vents, etc.
Windows are great source of heat, I tinted all my windows and made aluminized covers for the windshields. Huge difference in trying to keep the MH cool.
Window tint and window covers | 2001 Coachmen Mirada 300QB