I believe that the oil temp is usually ca. 10C about the engine temp, i.e. in 95-100C range. When towing 7,000 lbs, it is ca. 10C higher, i.e. in 100-110C range. I am not surprised by additional 15C when towing 7,000 lbs uphill.

I read that up to 150C, the oil should be fine.

I am sure that I will never own a truck... unless I will buy a farm, start cleaning or construction business.