Dash air vs generator

I think you would have a difficult time telling the impact on mileage from roof AC or dash AC. Factors like speed and direction of wind, hilly terrain, and time of year will have much bigger impact. We like to keep the whole coach cool, so if someone goes to the back, they will still be comfortable. If you are heading into the sun in summer, dash AC is usually required to keep cool. Climbing step hills in the heat may require turning off the dash AC if the engine gets too hot.