it is a 4x4 K1500 5.3L circa 2001.
vastly underpowered for towing a big Toy Hauler, but, I bought one of those engine tuners and reprogrammed the ECM for Torque using 87 Octane fuel.
it offered the option of 89 and 91 and 93 octane programs, but I just left it at 87 Octane.
the MPG varies from the middle 4s to the middle 9s, depends on headwinds, and the terrain. Always use cruise control set at 61 mph with OverDrive turned OFF.
in hilly terrain, have to use Tow Haul mode, and lock it in 2nd gear to keep the RPMs up in the 4,000 range
Climbed a few really steep grades in Colorado Springs area last summer, was in 1st gear and 4,000 rpm making a flock of Feet/Second