With a heavy vehicle (+extra heavy with your toad), you should manually downshift the tranny on hills and in traffic. Even with my little 9200 lb'er, I don't let it shift into 6th in traffic; heavy traffic I lock out 5th, just use the lower 4, and still downshift one or two as I stop. Downhill at 6% on a curvy i'state, I've been known to use 3rd for as much as 3 miles+, controlling the speed with the throttle, and using the brakes very little. If your tow-haul button locks up the converter, better still.
An acquaintance overheated her E350 class B's brakes just driving thru Philly traffic, no toad. She called in a panic because the steering wheel started shaking. Once I convinced her to let it cool and proceed in 3rd until clear of stop & go, all was fine.
Jim, "On the other hand, you have different fingers."