I have the same PC as yours, 2010 2551, and I tow a Jeep Wrangler. I use the "Tow Haul" feature because it changes the shift points so the transmission doesn't keep changing gears so much on hilly terrain. I think it is easier on the transmission because it doesn't have to shift so often in those situations. I especially like it when descending mountain grades to keep from overheating the brakes, it does a great job holding the speed on descents. When cruising on flat freeways I turn off the "Tow Haul". Don't really need to since the transmission will still go into overdrive in "Tow Haul" mode, but I don't like how it feels when coming to a stop in "Tow Haul" - hard downshifting.
I am curious about the transmission light problem, I went out and turned the key on and checked the manual and I don't see a warning light specifically for the transmission, can you describe it in a little more detail.
Bob