I can't do much about the heat and humidity in Texas, but here in California we have Death Valley. It has virtually no humidity, but it does get its share of heat. Earlier this summer it got to almost F 130.
Normally, I avoid that kind of heat for myself, my car, and my camera. However, I have done Death Valley in August, so I had to prepare for some heat. When I leave my camera locked in my car overnight, I try to leave it so that it gets as much cool air as possible so that it can last longer during the day. If I am forced to leave my camera in the car during the heat of the day, I keep it inside a cooler (not in contact with any ice). The ice will melt, but everything will still stay cooler than outside by quite a bit. When I am driving in the car, I normally leave the camera bag near one of the air conditioner vents so that just a little bit of coolness hits it.
If you don't like dealing with ice in a cooler, then there are piezoelectric coolers available that run off 12VDC power. The one I have is just about big enough for eight 12-ounce beverage cans, so camera gear fits it fine. The cooler will keep it at F 40, which is too cold for what we need, so I power it only about half of the time and it maintains about F 70. Yes, it helps to have a very good electrical system on the car.