I pull our '94 Bigfoot
17CB with an '07 Tahoe and use the Tow/Haul mode. Gas mileage isn't affected much, as with the 4:11 gears I only get 12-13 MPG without a trailer attached.
Last Summer before I bought the Tahoe, I rented a truck from Enterprise Truck Rental to go to the Black Hills. The smallest they rent is an F-250 TurboDiesel 4 door 4x4 rated at 21,000 capacity. I had to keep checking the mirrors to make sure the trailer was still there!