From previous time at auto companies, the trucks were tow tested (engine/trans cooling, heat damage, etc.) on Az Highway 68 east of Bullhead City. Trucks were loaded up to GCWR, with high profile trailers. The route was over 12 miles with average grade at 6%. In the summer, it's up to 120°F at the bottom and maybe 100°F at the top. Certifying with high profile trailers was a Marketing move. The SAE spec was GCWR with a lower profile trailer like a Haulmark and specified a minimum speed of 45 mph.
All of the auto companies used that route. The automakers had to maintain at least 45 mph with the SAE spec trailer in order to advertise GCWR. A/C shutdown, engine & trans overheating was not allowed.
I wasn't involved in brake testing, but knew some of the automakers used a mountain route in Pa.