I has similar problem on my class c. The ceiling is one big duct with holes cut out for the supply vents. The holes were not cut out all the way so I took the vent covers off and cut the holes with a utility knife. Air flow increased quite a bit. I think the person cutting the holes with a hole saw was about 2 inches too short to make a clean cut all the way through the ceiling. Next I took the AC cover off the inside of the coach and found a baffle that separates hot air from cold air. The baffle was installed incorrectly so the hot and cold air were blending. As far as temperature differential, a good IR thermometer will tell you if you have good cooling.
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