My truck has 215,000 miles on it and I calculate the mileage every tank full.
Its very consistent under various conditions. Loaded, unloaded, town and freeway driving.
The trip data in my car is 1 gallon better than actual every single time I calculate it. My truck doesn't have that info.
My old work truck was a 2007 Chevy 3500 dually regular cab with a 8.1L and 3.73 gears.
11 mpg empty and 4.5 mpg towing the same 15k lbs as the F-450 with 4.88 gears.
The F450 pulls better from a dead stop, "handle" the weight better but they climb the canyons towing at just about the exact same speed.
If I were in the market for a new truck I'd seriously consider an F-350 dually regular cab with that new 7.3L gas.
My truck is pretty well maxed out with my current camper but its certainly not worth the money to upgrade. At least for me it isn't.
Three years ago I went to OR, down the coast and back home over the Sierra's. The camper I had then weighed about 200 lbs more than my current camper and I averaged 8.7 mpg for the entire trip.
I have zero complaints about my current truck. Its the first Chevy I'v owned.