Whether or not you run your roof air conditioners while driving, you will want to start and run them prior to stopping at a rest area, checking in to a campground, etc.
My dash air is able to keep my 36' dp cool. I close off the bedroom door and try to stay ahead of outside ambient. I have been know to run both my dash and basement air while traveling in hot country. My Onen diesel genny uses about a 1/4 gal. of diesel per hr. Theoretically my dash air costs me 1/2 mpg but I have never seen it.
Most important to our creature comfort I adhere to the rule: It is easier, quicker and cheaper to maintain cool, then it is to cool a hot unit down.