I have a 2011 Ram 1500 QC pulling a 26BH. Usually the transmission temperature runs about 80 degrees above the outdoor temperature. For instance, on my last long drive (from Big Bend NP to Ft. Stockton, TX) the transmission temp was running at 138 degrees most of the time.
When the pulling is more severe, like on the uphill grade of mountain passes, I've seen the temp as high as 210 degrees, which is still not anywhere near overheating. I think 275 degrees is where the overtemp transmission warning comes on.
When towing, the trick is to keep the torque converter locked up as much as possible. That means operating the transmission in a low enough gear to prevent downshifts on most, if not all of the uphill grades you encounter. I always operate the transmission in tow-haul mode when towing, and using the gear-range lever on the shift knob, set the top gear to "4".
When in tow-haul and in 4th, the transmission of power is at a 1-to-1 ratio, basically direct drive. That will save wear on the gears.
Pulling up the mountain passes I will drop the gearing down to 3, or even 2, which increases RPMs and torque and lessens the strain on the engine and transmission.
My truck has about 37K miles on it now, and with about 25K of those being towing-miles, the transmission fluid is still bright red and clean and smells sweet. The owner manual recommends the first transmission fluid change (when towing frequently) at 64,000 miles, and if the fluid looks and smells good that long, that's when I'll change it.
2011 Dodge Ram 1500 HEMI Quad Cab
2011 Jay Flight 26BH