Tow/haul mode, yes, it's always engaged when towing. I have the 3.92 gear ratio and am not a speedster. I closely watch the temps on the vehicle information screen and check oil levels before and after each day when towing. But, and its a big one, we live in the middle of nowhere in the Finger Lakes of upstate NY. The nearest grocery store or gas/diesel station is 15 miles of highway driving. We do very, very, very little city or suburban driving. The only idling it does is when I am moving and hooking up boat trailers around the beach or moving the RV.
I love this truck. Torque, great mileage, luxury ride. Before new tires I got MPGs of 28 to 30 when not towing and 12-13 when towing, depending on grade and speed. Recent trip to Virginia along I-81 I kept to speed limit and got 33 mpg, hand calculated. I expect the MPGs to go down a little bit with new gripier Michelin tires.