Before buying a truck for the mileage, consider how much fuel you can buy with the $10k you'd save on a truck that gets 2 mpg less.
I had an old Ford Dually gasser .It was the last year Ford put carburetors on trucks , but I forget the year. Had it about ten years, probably around 1988 or so.. 454, 4 spreed, 4:10 rear end. Gong up mountain passes, fully loaded with 20 ft goosneck, I got 3mpg if it made it up the mountain without vapor locking. On an over all trip of 450 miles, two mountain passes I averaged 6mpg. Going up, I was mostly in 1st gear going 12mph. Best I got empty was 8mpg. I finally fixed the vapor locking with an ice bucket like racers use and an extra fuel pump, inline, mounted to the frame. After it left me stranded going into the tunnel on Loveland pass and I had the whole rig towed to Idaho Springs to replace the hydraulic line to the clutch .I sold it and bought the 98 Dodge Cummins, 5 speed.3:55 rear end. It averaged 14mpg over an all trip or 450 miles, two passes and 22 mpg empty.and usually topped the pass in 4th gear
Since then I know fuel injected engines get a lot better mileage, but I'd never buy another gasser for any type of towing.This V8 Duramax I have now gets 13 mpg fully loaded and 18-20 empty and handles up t 23,000 pounds GCVR
