Driving with the generator on

I'm from Florida, and most trips, most of the year involve using the air conditioner. I was told way back in 1995, when I bought my first Class C that it was more economical to use the generator to run the roof AC than it was to run the dash AC. I was told that a generator will use around .7 gal per hour. I almost always use the generator for air and not the dash air. I have noticed that when I was not using the dash air, the engines on all 3 of the class C's I've had ran a few degrees cooler.