The dash AC in our 24' C (E-350), is more than capable of cooling the entire coach so I've never even considered running the genny to get more cooling from the roof AC(just another benefit of having a 24' coach). The V-10 has plenty of power and we never hesitate to run the AC and I can discern no appreciable difference in mileage with it on or off.
At .5 a gallon per hour for the genny you would be reducing your mpg by about 8% assuming you are getting 10 mpg at 60 mph(we do). Given those numbers it hardly seems worth it unless your dash air cannot keep up. Either way it is really no big deal and being cool and comfortable is the only way I want to travel.
:C