Try setting the "Mix" setting when you run the dash air. This directs cool air to the windshield, the defroster function, and to your feet. We like using either def or mix when driving when it is warm {not real hot}, just turn the temp setting to full cold and crank the fan setting up as much as you like. The cool air coming off of the windshield and at your feet {the mix setting} we find more comfortable than the dash vents.
Another way to attack the heat issue is to use the dash vents {Crank up the fan as needed} and open the rear coach vent which will pull the ac through the coach more efficiently taking most of the heat with it. We live in southern Arizona and have yet to have any serious heat issues on our 2012 E-350 {V-10} 24' Class C.
Works for us.
:C