I don't think horses should be left in a sitting trailer, even if the windows are open. The heat WILL still build up. (The horses themselves are 1,000 pound heaters.) When the trailer is sitting still, there is no airflow unless you've got a very, very strong wind. There's only airflow when you are moving down the highway with the trailer.
Think of how small the windows are in a trailer, when you look at the trailer as a whole. Doesn't matter if it is a stock trailer or a slant trailer with drop down windows. You are not going to have good air flow when the trailer is just sitting there.
And as someone already pointed out, go stand in your own trailer for 10 minutes. If it's warm in there for you (at a measley 100 to 200 pounds with plenty of room to move around) think of how hot it would be for your large horse in tight quarters for a couple hours.
My horses are the last thing to go onto my trailer when we leave to go to a show (or go home) and they are the very first thing to be unloaded the instant we arrive at a show (or get home). I keep their comfort in front of my own. They're the ones who have to perform.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.