Eagle and APD started us on Tow/Haul mode. Another data point. 2013 GMC Sierra, 5.3 gas, 6-speed trans, 3.08 rear, HD Trailering Pkg. My truck has it and I've found it drops the mileage a couple mpg when towing for me.
When I head down to the Keys, f'rex, I'm usually running with the FF up around 6,400-6,500 and the truck/trailer totaling about 13,000. That's reasonably level but not flat driving, mostly on I95. With Tow/Haul engaged, the truck spends a lot of time revving in 4th and mileage drops to 8-8.5. Normally, I deadfoot at 55-60 mph in 5th rather than drive so it doesn't keep looking for torque (60 mph in 6th is about 1400 rpm). That keeps me in the 10 and maybe 10.5 mpg range. As an aside, in 2013, the Sierra 1500 got the same "powertrain grade braking" in normal transmission mode that in earlier models operated only in tow/haul mode; it uses engine torque to slow the vehicle rather than just the brakes.
I drove down without the camper a week ago using Drive and cruise. With the speed limit of 70, I ran about 70-75 and averaged about 19.5 mpg. When I could drop back down to 55-60, the mileage soared to 22. The best leg I had on that trip was down Overseas highway with practically no stops, running 45-55 mph and logging 23.7 mpg. (That's the best leg I've had in that truck period.)