The truck is essentially a parachute, the aerodynamic braking is rather effective, even going downhill.
Steepest I've come across would have been last summer on the Alpine Loop or a few sections on the White Rim in April. Never felt the truck was running away from me or I was on the brakes more than was prudent. That said, I wasn't towing and even when I do it's in the 3500# range offroad. We all build our trucks do what we want or need them to do. Everything is a tradeoff, in the end physics always wins.