Tow/haul WILL allow 8th gear, but only when the conditions are right. I've seen 8th at times when I'm towing but the speed I'm traveling and the grade must be just right.
My advice when towing is to forget about gas mileage and let the truck do it's thing. No use worrying about pennies at the expense of a transmission or your own safety.
Agree, I always let the truck do it's thing, in Tow /Haul or not.
Like mentioned above, in Tow / Haul, the truck has the full range of gears to select from, however, the way it chooses to select them is different, as it prioritizes economy, power, and refinement differently than the normal shift map.
MDS is disabled any time you have Tow / Haul on, or anytime you manually select a gear using the Gear Limit Selector buttons on the steering wheel.