Traveling in hot regions

Your dash air is far bigger than you roof air and will cool about 3 times as much. Up to 117 degrees or so we open a vent a small amount in the rear of the rig and use the dash air full. As you go down the road the vent will pull the dash air thru the rig keeping it very cool. Just be sure there are no other vents open so all air into the rig has to come thru the dash AC and all air out goes thru the vent in the rear. You will have to play with it to see how much you need to open the vent on your rig. Our rear vent is in the bath room so we can leave the rear vent open a little when stopped at a rest area, leave the rig engine and dash air on and run the bath vent fan on low. All air that comes into the rig must come thru the dash air. It works great. The inside temp in our rig is usually in the mid 70's
Also when running in the heat or pulling long grades, (As we do all the time) never come to a stop and turn the engine off. Leave it run until the engine temp is down to normal. For a 10 or 20 min stop just let the engine and dash AC run. If you engine over heats using it this way you need to work on the cooling system. If your dash AC does not cool your rig its because you have closed all vents and the air has no place to go so you will cool only where its blowing.
After about 117 degrees we run both dash and roof AC if needed.