When we had a TT and was towing with a GMC truck (gas, not diesel) the TOW HAUL mode was my best friend. Many folks think it just "holds the gear longer", but as stated, it changes the programming in the trans until you go back to "normal" mode.
Case in point: we camped in Colorado Springs a few years back, and after setting up camp, 1st thing we did was head up pikes peak. It was coming down that told ya TOW HAUL is a great tool. Not towing, just going down Pikes Peak, i hit tow-haul. Trans brake kicked in, and at the brake checkpoint, the guy hits my discs with his laser temp tool, and says "you weren't using your brakes?" I said, "nope. tow haul -- she crept down the mountain."
At that point, he laughed and said "guess you have to go back up and try again." We did, on a different day.
Also, as stated, when slowing down, foot not on brake, she downsifts slowing you down.
Great tool to use.