At those speeds you still have a hell of a lot of aerodynamic drag. I seem to recall increasing speed from 60mph - 75mph (25% increase) Aero drag increases 56%, 70mph - 75mph (7% increase)aero drag increases 14%. If you can reduce speed to 55-60 you should see big benefits. Next thing is to reduce gross weight, remove unnecessary junk from RV and sailbox. Good vehicle maintenance goes without saying.

Cheshirecatman