I wouldn't trust the trip computer cuz mine says I average between 20-23 mph best way fill your truck up drive it x number of miles and divide miles by the number of gallons for accurate mpg. I'm getting about 15-17 in town and 17-20 all highway.
I have a stock 07 5.9 QC 4x4 auto and hand calculated I have gotten 19 to 20 on long road trips cruisin bout 70, towing my 19ft toyhauler up 6k ft in elevation in 180 miles I got 10.5 mpg at 65. The toyhauler loaded is about 9k lbs. This winter I am lookin foward to see what kinda mileage I can get goin to Glamis which is mostly flat.
The best I've gotten was a little over 22 MPG trying to stay at just 65 MPG for 4 hours straight (hand calculated) and then another 4 hours back home that day. It's stock...haven't bothered looking at the MPG when pulling the trailer though.