To give you actual numbers if I run two roof top AC's off my genset it will burn about 1 gal an hr, if you travel 60 mph and get 8 mpg it will reduce your mpg to 7 mpg. Having said that my dash AC is sufficient to keep the front cool for driving during the heat of the day. I have a curtain that I used probably twice in 15 years for really hot days. Out west once stopped for the night the temps drop pretty quickly because of the higher elevations and never use the AC. I don't go south in the summer only winter. The TV, AC's and Genset are the three things that rarely get used.