Truck for Eagle Cap 1165?

I am running an 08 F550 with the 4:88 gears. My curb weight is about 15,000# and the 6.4L Ford PS only gets us about 8 mpg which I feel is too low. I will be looking at programming and other modifications in the spring to improve on this mileage. With all of that said, this truck pulls great. I tow an enclosed tandem axle trailer loaded with a work bench, tools, generator and an 87 Jeep rock Crawler and I towed this setup over Cottonwood Pass in Colorado last August. I am careful with my equipment so when we were on the extremely steep inclines I ran about 20 mph. No sense in stressing that trany while I was out in the middle of nowhere.

The newer trucks have an upgraded transmission with more gears than I have. I believe my '08 is still running a 4 speed