Tow-Haul locks a transmission out of overdrive. If you are moving on flat land and the transmission is not downshifting and then upshifting (hunting) then you do not need tow-haul to be engaged (which disengages the OD gear).
If you are traveling downhill and need a bit of engine braking, then engage tow haul mode. If you need more braking, downshift.
Leaving your tranny in TH mode all the time when not needed will lower the mileage.
36' Itasca Meridian DP, now 2005 Newmar Scottsdale 34' Gas