When we had a 24ft class C with factory air. The dash air was able to keep the cab area cool. I attributed this to the smaller glass area of the windshield and cab over bunk shielding the windshield from the sun and reducing the area to be cooled. Now in our class A there is no way that the dash air can keep the cab area cool. I attribute this to the very large windshield. We always run the gen and usually only one of the two ACs. I learned long ago not to worry about MPG when driving our house down the highway. It is what it is.