Ford F350 axel ratio 3.31

I have a 2011 F-350 SWR 4x4 w/6.7 and on our first trip empty we went 325 Interstate miles @ about 65mph and dash read 20 mpg. On return trip of about 400 miles, mostly on state highways towing a 38foot fiver the dash read 11 mpg., speed ranged from 55-65 mph. The truck only has 8000 mi. on it. I'm told not to expect the best mileage until we get around 40-50,000 miles on the engine. We're running 3.73 gears.