175,000 miles on my 2002 Sierra, did a moderate amount of heavy towing, much hauling, quite a bit of hard driving, and a moderate amount of rough terrain...Look at my gaslog. Average 18.5mpg. Before I knew anything about hypermiling, in the winter when I was using the remote starter daily and idling a whole lot, I was getting 15.5mpg.
Your 10mpg is definitely because you only ever use it for towing and wheeling.
Come to think of it, I got 10 to 12 mpg towing my 6000 pound 26' camper.
The Beast (1998 GMC Sierra extended cab 4x4) has 151,000 miles on it, and I've been running full synthetic oil in it since I bought it 4 years ago. It runs flawlessly, and when I took it in this spring for the 150,000 mile service the dealer said it ran just as well as a new one. In my normal mix of driving I usually get 13 - 14 MPG (without hypermiling), and that's with a good bit of stop & go city driving.
I know the newer ones get better mileage, but as much as I'm driving now I don't think its worth it to replace it. What I may eventually end up doing is selling Rusty, make The Beast my beater pickup truck, and get something that gets better mileage, and is still comfortable. Maybe something classic and fun like a Pacer or a Gremlin. I like the looks of the old Ramblers and Studebakers, but getting one with factory a/c is going to be hard. Our old AMC Spirit got great mileage, but we didn't keep it because it did not have a/c. I think that was the only car my family got rid of with less than 100,000 miles on it. My mom's old 88 LeBaron got great mileage, had an in dash mileage computer, and was pretty comfortable for a small car. Even with my mother driving it the car averaged 25 MPG. I imagine it probably would not be hard to hypermile a K car into the 30's.