I don't drive my 04 QC 2wd 305/555 long bed much, so rarely check mileage. But last week I did a 700 mile round trip to Oregon, twice. Both times I got near identical mileage, 19.5 empty, and 16.4 flat towing a 3800 vehicle. This was going over some pretty steep mountain passes, and cruise control not working.
I was by no means babying it, although I-5 in northern California is crawling with CHP's. So, with a speed limit of 55 towing, I rarely went much over 63 mph in that 50 mile section. The rest of the time I was mostly doing 65-70.