I am not sure what my towing temps are yet as I have not towed with my new 2013 F150 Platinum, but I do know that I have been running in the mid to upper 190s on highway trips since I bought this truck in 11/13.
At 195, the needle on my analog temp gauge is not even at the halfway mark on the gauge.
I found the following forum post where there was the following quote:
Transmission Temps when Towing
This is what I was told from the service manager who spoke with the Ford service hotline regarding my concerns with tranny temps. FMCO said: "There is an inline thermostat like restrictor that prevents fluid from going through the tranny fluid cooler until a certain temp is reached. This thermostat starts to open at ~198º, and should be fully open by ~203º, allowing an unrestricted flow of tranny fluid through the cooler."
Assuming this is true, I am interested to see what my tranny temps will be this summer when it is in the upper 80's/90's. My previous TV (2011 Avalanche) ran in the 160's not towing and 190's to a high of 206 when towing.
I am guessing that the high limit on the temps varies by vehicle and more importantly the ATF that the vehicle is using. So, I would check with your vehicle's service manager for specific specs regarding the year and model of your truck.