I'm not a transmission guy but I do know by using the tow/haul mode your torque converter will lock up sooner and provide better engine braking. My 2010 v10 in a class c E450 with the 5star tuner stays in tow haul about 1/2 the time. I live in Nevada and drive in the mountains a lot. I tow a Tacoma pickup at 3700#.
Retired RV Technician
Travlin the West
Winnebago Outlook, Victory Vision, Silverado 3500, Dog ,Wife ( not in that order)