We generally try to avoid leaving our horses in a trailer (unless an emergency occurs) when the temperature is above 80-85 degrees (depends on the humidity). Iowa is very humid in the summer, and can be a complete nightmare to walk in, let alone stand in. We have a big, fairly airy stock trailer that we use to transport horses in. One time (a day of about 85 degrees; very pleasant at the time) we loaded 3 of them up and took them 20 miles away to the lake. It took all of 20 minutes to get there. They were all 3 completely COVERED in sweat, and very uncomfortable when we got them out.
This past 4th of July I saw someone put 4 horses in their stock trailer after the morning parade, and leave them there for 2 & 1/2 hours. They had been squeezed in there, as it should have been a 2 horse trailer only. Very, very little airflow, high humidity, 100 degree temps, and not even parked under a tree for shade. Those poor horses looked miserable, and I called the police on them. The police took an hour to find the driver/owner, and he was going to let them stay there all day until the fireworks were over (10 p.m.). He showed a total lack of interest in his animals' well-being, and was mad that he had to take them home and come back (it was a good 5 hours before the fireworks at this point).
So many people around my area do this without even thinking of the conditions. Gives a whole new meaning to the phrase, "Rode hard and put up wet."
And I know of some people that tie their horses to the outside of their trailers as well. Yes, your horse may get hot there as well. But if you angle your trailer correctly, eventually the trailer will provide some shade to your horse. That's what we do, and it works great.
I've also seen horses refuse to enter a trailer again after having to stand in it, in the heat. You might create more problems than it's worth.