my bus has always been a 7-9mpg bus. i drive with the foot to the floor all the time. that gets 60-67mph.
on my first trip this summer, i caravan-ed with another skoolie, at about 5mph slower than foot to the floor. 55-60
i didnt get to check my fuel mileage, but the needle never moved off of full for the 2 hours or so that we drove together. after they turned off, i sped up and the gauge started moving like usual.