First, what is the frontal area of the travel trailer?

Ford says not to tow a trailer with a greater than 60 sq. ft. frontal area, regardless of trailer weight. (Also worth noting, for a trailer of that weight he needs to be pulling it with either a weight distributing hitch or fifth wheel.)

After that, I would be looking at the trailer's axles and brakes. Make sure the axles are straight and properly aligned, make sure the wheel bearings are good, make sure the brakes are good and properly adjusted. Anything binding or out of alignment could drive temperatures way up, even if it's not binding enough to make it noticeable to the driver.