I know the guys who designed that tranny/engine combination. First of all, Tow/Hawl mode changes shift points, releiving stress on the transmission. This might seem like it will consume more fuel, although they insist that it should save fuel. Their explanation: The RV is heavy enough to be considered as a vehicle towing. I tested in several different units, and the difference is minimal, and in the lower side.
Second, the transmission will allow you to use the engine to slow down, which is in my mind the best advantage of this transmission.
All that said, It has been proven that using the tow/hawl mode will prolong the life of your transmission. I have seens trannys last over 300K, as rentlas, as long as the default is Tow/Hawl mode is ON.