I grew up in the mountains of West Virginia. One trick we had with our old cars was to run the heater, even in the summer, going up the mountains to lower the engine temp and prevent boil over. I guess thats why I'll run the roof air and keep the dash air off. I still worry about overheating. Plus I don't mind the cab a little warmer and the co-pilot likes it nice and cold ( freezing ) in her domain back in the coach.
I've read that the genny only uses a quarter gallon per hour. I don't know how the dash air affects mileage but it has to have some drain on engine rpm same as all the other accessories.
My generator does not kick off at a quarter tank. My low fuel level light came on one night on the return leg of a trip while the generator was running. The co-pilot was asleep in the back and I didn't notice how low I was on gas. It took over 30 gallons in a 33 gallon tank and I had to shut the genny down. It did not shut off on it's own.