I've been pulling my brother in-law's camper, weighing in at around 9k lbs. and I haven't had an overheating problem yet. Perhaps theres another issue going on? Just curious.
Oh and BTW, I'm in central Texas. Last time I pulled the camper it was around 101 degrees outside, in the shade, still no overheating problems.
Oh and if you're ever driving through Arizona and your motor is overheating, just turn the AC off (shouldn't need the AC in Arizona this time of year), less pull on the motor.
Grizzy