I get that you don't want a diesel, and that's fine. But since you seem to be a mileage junky, I'll mention this. My 2000 dodge 2500 24 valve 5 speed cummins will get up to 28 mpg on highway trips even loaded with no trailer. I use a scan gauge II which pulls info from your obdII and this scan gauge is known to be quite accurate. Now when I pull my 20ft enclosed weighing in at 10,000 lbs and driving through the stop and go city traffic, the mileage is 12-13. I am 26 and no light foot either. My repairs have been limited, and IMO if you are a mileage junky, the 5.9 cummins is the only truck to have. Imagine if you only had to pull that little trailer around with a truck that gets incredible mileage.
Walker, Toro, Stihl