I live where the summers are very warm, and can be quite humid (although not extremely so), and also prefer a trailer with stock sides to fans. I try to haul early or late, and hose the horses down before loading in extremely hot weather.
I haven’t done the ice in the shavings/wet the shavings trick (although I’ve known of it), since I’ve found that the breeze from the side openings disturbs bedding to the point where I prefer to haul without bedding, to keep it out of my horses respiratory systems and off their coats, etc.
Also, I’ve heard, on local TV news, that fans are detrimental in very high temps, as they cause more of a convection oven effect than actual cooling. From what I remember, the tipping point was about 95 degree +; this tidbit was mentioned during a feature about fans (there are frequent “fan drives” here each summer to provide free fans to those living without AC). The gist of it was that, at high temps, the fan is simply blowing very hot air around, and doesn’t help cool.
The only thing I was able to quickly internet-search, somewhat on the subject, was this: https://theconversation.com/electric…heatwave-64834
I realize that horses are not elderly humans, but since learning that fans are actually contra-indicated for humans in certain circumstances of high heat, I’ve decided against having fans installed in my horse trailers. The last thing I want to do is turn my trailer into a convection oven and cook the horses!
In the past, I’ve had solar screens installed on the openings of a stock-type trailer, and that seemed to cool the interior quite a bit.