This is difficult to diagnose without having the truck/trailers in front of me. I would be looking for some sort of pattern in the vehicle's behavior and operating temperatures.


Having had basically every mechanical item replaced, it sounds like the issue is either caused by something he is doing wrong or is caused by something software related.

On a side note, he is making sure to use tow/haul mode while towing, right? Towing a heavy trailer in the normal fuel economy mode is a great way to roast a trans. Also, he shouldn't be towing with a new/rebuilt trans until he puts 500 miles of mixed driving on it (break-in period).