I've only been on a few log trips with my bus, about 2 years ago we went to South Padre Island TX for spring brake with it - and I documented my fuel usage. When I got home I put it into the table below. My speed-o is off, it reads 11% slower than you are actually going. I figured this amount by calculating the mileage mapquest stated vs. the mileage my odometer read. The mileage in the right column is the corrected fuel mileage.
For the entire trip I averaged 8.3 MPG
On the way down I followed Jason's bus which maintained his top speed of 65mph. At that speed I averaged around 9mpg (one tank was almost 11)
On the way back, we left before Jason and traveled at my bus's top speed (70-75mph) and got much worse fuel mileage. One tank we got as low as 6.17MPG - this was driving at wide open throttle for an entire tank of fuel. A few tanks later we went a steady 55mph and got 12.89MPG.
Speed, wind direction and terrain all play a huge factor in fuel mileage. For planning trips I estimate mileage at 8mpg to cover my fuel expenses, and I try to drive 60-65 mph to get the best mileage. If I drive it right I should be able to get 9-10 MPG.
My bus has a cummins 5.9 turbo & innercooled with AT545 (I think) and 4.33 gears. Also it's got the aerodynamics of a barn.