I moved 2000 miles in a 20' penske truck. Avg mpg was around 6-7, going no faster than 65. This was really loaded, and towing a double axle trailer hualing my car, going from CO to NY. So, YMMV. I wouldn't count on better than 8 though.
This was 2 yrs ago. The truck was much nicer by far than the Uhauls we looked at. Truck was almost new, Vortec V8, etc. Had one prob however, tires on trailer were nearly bald, and one blew around St. Louis. Penske relatively quickly got someone out to fix it, and gave us a discount when we turned the truck in.
Hope this helps G/L
PS. Inspect those tires before you go!!!!