I gotta chime in to also say...I run the rooftop A/Cs with the vents/windows/shades CLOSED except for the front windshield curtains. On a hot Southern Summer day of 95 degrees plus with humidity ranges in the high 80%-90%, the dash unit won't even keep the driver's cabin cool enough for me...much less the house. I have two rooftop units, one 13.5K and one 15K. I run both, in addition to the dash and run a small fan pointed at my driver's seat to stir up the cool air from the floor. When I get to my destination, my coach is nice and cool and there's no struggle to get it "climatized" before the camping fun begins. As for fuel mileage, on a 6 hour trip, I lose about 0.5mpg running the genset....plus it gives it a good exercising under load and I don't have to burn LP to keep my fridge cool.
