Sorry to hear about the hassle. I drive a similar truck and have not had any issues towing 8,000 down some pretty good grades. You may want to double check your trailer brake controller settings. After following Fords instructions on the trailer brake controller setup I have always felt the trailer was taking care of itself and was very impressed with the overall braking. The longer downhill grades are about the only time my engine is over 3k rpm and it does okay at helping slow things down. It does not 'drive itself' and is not as good as my old 8.1 with the 4.10 gears, but overall it has done well. I try to begin my descent at the speed I want to be at, coast on the engine braking, use the pedal to brake the speed down a bit, coast, repeat until at the bottom of the hill.
Good luck,
Stu