Going down to Pittsburg Landing on the Snake River is 16% for 6 miles, graveled dirt road.  Pulling a 7000lb jetboat down that road required 4-low, and if you had surge brakes you had to lock them out with a bolt at the top.  Sitting in the campgrounds at the bottom I saw many sets of brakes smoking.  Once saw a guy come down the road, and backed his boat right in the water, shatter all four brake drums on the trailer.
I smoked a set of brakes on a mid-90's F250 by coming down a steep dirt road with the camper. (lighter one than I have now).  Boiled all the fluid out of the master cylinder too.  I would guess the OP didn't use 4-low, (if available).