For the larger motorhomes it may be best to leave tow haul on all the time. For us, with a 26ft E450 B+, it seems like the tow haul reduces gas mileage because it shifts down more aggressively and stays in the lower gear longer. I try to keep ours in overdrive as much as possible and only use tow haul as needed.
Tow haul does seem to give better performance because it keeps the rpms in the power zone more of the time.
I have no idea of any impact that the use or non-use of tow haul has on engine or transmission longevity. I assume it makes no difference.
Just one persons un-educated opinion.