I use tow haul mode all the time for two reasons. First, it prevents excessive braking, especially when headed down hill. Secondly, it helps to make the motorhome stop more quickly as you can press the brake pedal and force a downshift. From what I've read, the transmission can handle it. I just came back from traveling through a lot of the mountains in Pennsylvania and when downshifting, the RPMs usually stayed at 4K or less. Without using the tow haul mode, I think it would be very easy to overheat the brakes and damage them.
Years ago, I didn't really understand the importance of downshifting when towing a trailer with a Dodge dually (gas) and I ended up having smoking brakes and had to pull over to let them cool down. Obviously, it wasn't good for the brake pads.