I may try switching to a different fluid, it is due for a tranny fluid change. After talking to several GM service techs, it does seem normal to be in the 185-200 range when towing. I did notice, after cleaning the heck out of the radiator and every other cooler, that the transmission is running much cooler. I pulled our Troops scout trailer this weekend, which is not heavy by any means, roughly 2500lbs in a 6x12 cargo trailer in 95 degree temps and the highest temp from the tranny was 185. I have in the past pulled this same trailer and have seen the tranny hit close to 200 in cooler weather, I really do think the cleaning has helped tremendously. I won't really know for sure, until I have this 5000lb, almost 26' ft long trailer behind me.
2013 Starcraft Travel Star 229TB
2011 Jayco X17Z(traded in)
2010 Jayco Jay Series 1207(traded for the 17Z)
2011 Chevrolet Silverado 2500HD Z71 CC 6.0L/3.73
Reese "Pro Series" 800lb Trunnion WDH w/sway control