I've done my far share of towing around the country in the last few years.
I can get 11+ mpg at near max gross weight on a 2005 4x4 duramax, going 70 mph with average hills. I go slower than that my mileage actually drops, faster than that and it drops. This last trip to the Hammers I had one tank at 9 something as the low and two tanks at 11+ as the high. Several more fell into the 10 range.
Like I said, count on 10 mpg. Maybe a bit higher if you have a two wheel drive but I wouldn't count on it.
If your coming that far get a cabin with shower and plan to eat food you cook to help save a bit of $$. Definitely stay for a while, there are nice site seeing things around Moab that don't require wheeling.
You guys are running a programmer, my 04 is almost identical (LLY with ZF 6 speed) to yours and I average about 8.5-9mpg with a stock tune and a similar load (cabover camper, trailer, rig). That being said I plan on having a programmer by blazer bash this year. I'm guessing diesel will be over $5 a gallon again.



