Obviously a bugaboo for motorized RVs, the larger they are the worse it is for driving in triple digit temps. All the ideas mentioned above can have some effect for some, more than likely not dramatic enough for most. We purchased our current class A in Texas and picked it up in late July enduring record temps that hovered around 115 degrees.
Previous owners replaced dash AC in hopes of making it more bearable, indeed air out of vents was really cold but the time it reached us in cockpit seats it simply did not do the trick. You can throw a bunch of money at it with minimal results, kind of like trying to get better mpg, best and most effective method is to just drive a bit slower.
We found if we just left a little bit earlier when temps soared by noon we were at least accustomed to it, thankfully my DW and I were born, raised, and lived most of our adult life in Phoenix and are able to tolerate heat somewhat. Hopefully you find a method that at least makes it more bearable, I know it would be helpful for so many with same issue.