I borrowed my parents truck, a 2006 2500HD with the LBZ pulling a similar but slightly shorter and slightly lighter Wildwood last summer for 4600 miles. We got just shy of 11 mpg I believe (hand calculated) over the trip running 65 at a minimum, usually 67, one day 70 for much of it. You must have some sort of issue but I don't know what. My 8.1L Suburban gets 7.5 mpg at 65 mph.
I ran the LBZ last summer in 6th all the time while cruising. We went from MI to Glacier NP and it cruised in 6th at 67 mph the whole way and never downshifted until we were almost at Glacier. We're less than 8k loaded.
