Just a WAG here. I think vacuum decreases with engine loading so perhaps when you are loading the engine actually driving on the freeway (especially going up a hill) vs sitting in your driveway, the engine vacuum drops and that may be enough to affect the vacuum operated blend door in your dash A/C unit (or a vacuum motor, etc.). Maybe a deteriorating or leaking vacuum hose connection somewhere or leaking vacuum check valve. If the hill is a long one you might experience a temp shift in the coach depending on outside air temp etc. I would also expect coasting down hill (throttle body closed) would raise engine vacuum and cooling would return to "normal" if there were no leaks. Not sure if there is a vacuum reservoir on these coaches.
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