Insulating the roof is a good idea. Also installing roof vents helps a ton with airflow. My trailer has the kind that you can either open facing front or facing back, so you can decide how much extra air you want. All summer I open them facing front, but it is nice to open facing back in late fall/winter/early spring when it cold but I still want air moving for respiratory health, but maybe not direct flow on the horse.
The only problem I worry about is if I were to get stuck in a traffic jam in the summer. Vents/windows won't do much if you're not moving.