Drove my wife from Des Moines to Kansas City and back today so she could pick up a bird she bought. (Don't ask details about the bird because I don't know and don't care. That's her thing.) The round trip was 405 miles, all interstate except for about 10 miles. With the truck as described above, cruise control set on 67 mph on the way down and 72 mph on the return trip, it got 18.5 mpg. That's figured on miles driven according to my odometer which matched the mile markers in Mo. almost perfectly, as well as Google maps, and gallons of gas used.
Interestingly, it got better mileage on the return trip at the higher speed, 19.3 for the last 145 miles vs 18.1 for the first 260 miles. I thought I had set the cruise at 72 going down, but realized it was set on 67 as we got to the north edge of the KC Metro area. I might have noticed sooner, but that was about what traffic was running so it seemed right. Traffic was faster all the way home, but I ran up behind semis and had to slow up to let traffic go by several times. Didn't really have to do that going down. There was a light wind from the south/southwest, but not enough to make a difference. Maybe the truck liked the gas I bought in Mo better than the Iowa gas.
Thanks for the update!