I've never driven a MH of any kind (I'd love to some day), so here's my worthless suggestion. Why not just deflate the tires to 75 (cold) and take it out for a test drive. As you've indicated, the tires heat up quickly, so you should have your answer within a few minutes of driving. Take it out for a 20 mile spin and see what happens.
If it doesn't work, then simply air up the tires to 80 psi again. Nothing lost except 20 miles of fuel.
Is the 80 PSI the "Max" cold PSI? I know motor homes are a different animal than the family sedan, a dually truck, or a trailer, but somewhere there has to be a "sweet spot" that works for your unique rig, tires, and overall configuration.
