That's exactly right. What people seem to forget is that it's also not just about how far north you go, as even some places in Canada are still unbearably hot in the summer if you're living in a van. What I have found is that it's more about going up in elevation than it is about going north. However, a nice mixture of both should be helpful. I also want to escape the humidity more than the heat because the heat can be managed whereas the humidity cannot.
For every 1,000 feet you go up in elevation, the outside temperature comes down 2-3 degrees. That may not sound like much, but if it's 100 degrees in Dallas, and you're at 550 feet above sea level, if you go up to 3,000 feet elevation, you will have brought the temperature down to a bearable 92(ish) degrees.
Still a bit warm but here's where you can get crafty :idea: . If you park in the shade, you can bring down the temperature by a possible 10 degrees. So now you've got it down to a near 82 degrees. Couple that with a solar panel on a leash, and put in the direct sun, a 50 watt fan to bring you air circulation, you should be comfortable as long as you be still.
I learned this trick by staying in the Cheyenne mountains in Colorado Springs while it was 95 degrees in the city. I drove up the mountain for about 4 minutes (roughly 3500 feet higher where I parked than down in the city), parked in the shade, deployed the panel, ran a fan, and BOY what a difference it made. My inside temperature gauge red 76 degrees and the humidity was only 31%.
Plus, the view was amazing.