I think it depends on what kind of heat you are battling,is it a dry or humid and what temp is comfortable/conditions to you.
Dash air with top windshield fans running will help the front 2 people keep comfortable in moderate heat. Some folks will add a curtain behind the seats to isolate the area.
With all shades and curtains closed but driving into the sun &/or having pets or guests onboard...all bets are off you may need the roof AC units as well at least part of the day.
Last summer/the hottest on record, we traveled in the midwest and SE, left in early June and returned in mid-July, temps at 100 the whole time...we had to run the roof AC units almost every mile or our 3000+ mile trip. What a bummer so I fully understand your concern! That was the most miserable trip we've ever taken, well except for the year we were in Las Vegas in late June!!! This year we are heading out in the fall!!!