i have a 2002 duramax allison purchased new .
like most others i have discovered how to get the tranny to downshift for braking using the tow haul mode button and moving the lever to 3rd or second .
i live adjacent to the mighty cascade mountain range at 4100 ' ASL .
you live in Lew-Ease-ee-anna at see-level .
your braking problem is for this trip only .
my suggestion would be to study the routes and altitude changes you will encounter each day and just go slow down the steep grades .
my buddy with a nifty cummins chose to replace the brakes on his fifth wheel with disk brakes and also 16 inch wheels so he can use LT tires with 3042 # capacity .
but his trailer is the heaviest / largest within our circle of rv friends .
for one trip , the best plan is the slow lane , downhill .
