I never have problems hauling in that kind of heat, but I also haul in a stock trailer, so the horses are actually more comfortable moving in the trailer than standing outside of it. An hour and a half shouldn't be a problem for you, especially since you have a camera, so you will be able to tell if they are too hot. My biggest concern is how open your trailer is-- I've seen horses arrive at shows in closed up trailers, and almost without an exception they climb off those trailers sweaty and hot. A horse itself generates a lot of heat, so closed trailers aren't a very good choice unless you are hauling in very cold weather. I would open or remove every window you can safely open in that trailer, and let the wind as you move down the road do the job of keeping the horses cool. I would also offer a nice grass hay in hay bags for the horses, so they can relax for the trip. I would probably bring water from home, since the horses are more likely to drink water they are used to.
As others have said, I would not unload them unless you are certain the trailer is too tightly closed and they are sweating a lot; standing out on a busy, strange road in the sun on a hot day isn't going to cool them down much, and could make them hotter.