I use tow haul whenever I tow my TT. The engine speeds are higher providing more power. The transmission torque converter stays locked up more often. I think my manual says it is only recommended at 80% of GCWR, but the huge frontal area of a travel trailer has more impact than weight for RVers.
I also use M5 for highway cruising. The trans does attempt to hold 6th gear with my combo at 65mph but cannot maintain speed. 5th gear 65mpg is about 2200rpm, not very high given peak torque is over 4000rpm and hp 5500rpm.
For the 5.3L/6-spd/3.42, you may want to limit gear to 4th or 5th. Frequent shifts generate extra heat in the trans. 4th gear should still be under 3000rpm with good throttle response and fuel economy is likely the same as 5th gear it your truck can hold it. Do some experimenting.
