Agree with Danny. GMC is known for close coordination with the DuraMax & Allison in Tow/Haul mode. I've never towed without it the whole time. Some people use it selectively because they believe it reduces fuel economy but I always leave it engaged.
Personally, I'm still amazed how my GMC manages most of the towing decisions, freeing me up to make all the mistakes... Really love this truck!
Mike & Pam '12 36RL, Kodiak Discs, 17.5" G114s H-Rated, SRE4000 w/HD Shackles, Joy Rider Shocks, Flex Air, 5.5 Onan, W&D, DW, 4 Scare Lights, (2) 6V Trojan T-145s; Progressive EMS-HW50C, Winegard DTV Sat, Sony/Denon 4K Home Theater, WiFiRanger EliteAC Pack, weBoost 4G-X Cell Booster.
2020 RAM 3500 Limited CC, LB, 4x4, DRW, Cummins HO, AISIN, 4.10, Curt A30 w/OEM Prep, Transfer Flow 70gal Aux Tank/Toolbox.