The stock gauge in the 03 and later trucks is worthless, it will take 280 degrees to show close to the red!! By then your trans is toast!! Every auto used for towing should have a good quality, accurate trans gauge. As for the 4R100 the correct answer is 70-90 degrees over ambient is normal. The lighter if unloaded and the higher when towing heavy. If you have sustained temps over 200 I would want additional cooling, 220 is my pull over temp. I just towed my 10K TT in 95 ambient and the warmest I saw was 160, I am running the 6.0 cooler and that's it , no OTW cooler in my truck!!
NCH