I would not adjust the brake controller based on steep declines. As others have said, the engine breaking from your truck will do most of the work. I am towing with a 2011 F350, we just drove from Rapid City, SD to Glacier National Park and just got to Yellowstone today. Even on the steepest inclines, I only had to tap the brakes a few times to keep the speed in check, the engine breaking took care of the majority of the work.
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