A couple of thoughts, as I believe you have the same transmission that is in my Super Duty;
1. You're not supposed to manually downshift it, it does that on it's own.
2. Try tapping the brakes when you feel you need the transmission to kick in and help with the braking. I have found on my truck if I just let it go it provides some engine braking. If I just tap the brake pedal once the engine braking becomes far more aggressive.
Hopefully that will work for you as well.
Please no more suggestions on how to drive. This has all been discussed on the irv2 Ford forum under a half dozen different threads. It is not the same transmission as the Ford super duty, but the tow haul feature should work the same way. I traded a 2011 F53 in on this unit and the tow haul feature worked as described in the owner's manual. I currently own a 2013 F150 and the tow haul feature functions identically to the 2011 F53. I have been down 6-8 percent grades in the 2016 F53 with tow haul engaged at least 50 times usually various braking techniques and the transmission never downshifted once. Other owners have had similar experience with these units.
