Well actually, towing in O/D can raise you temps. Basically, what I have experianced, when the trans starts to search for gears, like shifting between drive and O/D, that creates heat. Plus, O/D isn't made for that type of stress unless you have had it rebuilt.
As for temps, with the stock trans,in good health, I would see about 200-210 going up hill.
A good piece of insurance, is to get a larger trans cooler.
Also, when I tow in the mountains, I take over and shift for the truck to prevent it from searching. Most of my uphill is done in second.
Also, with sifting down, your exhaust temps stay lower, i know it sounds funny, but it's true.
Plus, my 5th wheel weighs in at about 14,000 lbs.