If you crack open a rear vent, especially if you can run the fan on a low "air out" setting, or window in the back of your coach and run the dash AC on high {aim the vents straight back and not in the face of the passengers}, it will go a long way toward cooling the coach.
In all but the most extreme temps this often will be all that you need. On a large Class C, 32' + obviously this won't be as effective as one that is 32'or less but it can often eliminate the need to run the generator to feed the roof AC. Running your generator will work but is going take at least half a gallon an hour and often more than that which will hit your mileage/wallet.
:C