Tow/Haul mode came out in the late 2000 on many gas vehicles. It changes the tranny shift points and when the torque converter kicks in. Unlike the OD feature of the past this is computer controlled. Awesome feature to have.
Tow Haul mode does act like a engine brake when going down hill. I prefer to leave it on and just use the brakes occasionally to slow the RV down. If you don't like the high RPM's turn tow haul off and plan on using your brakes more (which can reduce there life and warp rotors).
If you are on a hilly road, tow a car, heavy winds or just fully loaded I would also run the tow haul mode as it will let the engine rev a little higher in gear which is good in those conditions. Light load and tail wind you may not want it on.