Ok. Then IOKKI has the right numbers for your vehicle after all. The highway mileage difference may be due to the California emissions package, which the 49-state versions (obviously) don't have, and which probably rob you of at least 10 horsepower. I'm betting you still only get 8-10mpg in the city though, even with the 350 (5.7liter). You might consider trading up to a 3/4-ton, with a diesel, especially if you tow. The 1/2-ton can tow that 2-ton horse trailer, but your rear springs and transmission will not thank you for it (you do tow in 3rd, not overdrive, right?).
"you do tow in 3rd, not overdrive, right?"
Yes, I don't like the way the vehicle "seeks" between overdrive and 3rd.