While most are commenting to you about the equipment possibilities or things to check, I wanted to let you know that they way you drive while towing is also very important. I am on my third Dodge (now Ram) a 2013 diesel 6.7 with the 68FE trans.
From previous set up, I had exhaust temp guage which goes hand in hand with radiator temp. In order to keep the truck temp cooler I had to balance the exhaust temp and boost from the turbo. I rant the temp at around 1000 to 1100 which and of course using the correct gear to maintain RPM. It's a balancing act and it kept the radiator cooler, thus the trans cooler as well. If I ever put my foot into it, the temps shot up, so I learned to cut back. I am pulling, not racing and it chugs up all the way. Try the 16 mile Baker grade near las Vegas on 116 degree day pulling anything with never a hiccup. So, my advice also to find your sweet spot learn that pull truck and you will be fine.
