When in Tow/Haul mode, our TorqShift transmission (used by Ford in motorhomes from 2005 onwards) does not lock out overdrive. Overdrive still kicks in just fine with the TorqShift transmission.
Since a motorhome is like a truck that is nearly fully loaded all the time, it might be better to stay in Tow/Haul mode all the time. This provides for better and safer driving in the mountains, and when left turned on all the time ... just might provide slightly better gas mileage due to less overall transmission slippage. Transmission slippage wastes gas and heats the transmission. I have no proof on the gas mileage aspect - it just seems intuitively true.