That sounds like a great (and very long) trip. When you think about it, 8.5MPG for an 8 or 9 ton vehicle that has the aerodynamics of a barn door and can move 63MPH really isn't bad. On a pounds per mpg basis, that's 1882 pounds per MPG (at 16,000 pounds GVW). An economy car that weighs 2000 pounds and gets 35 mpg is only getting 57 pounds per MPG. The efficiency of the bus is pretty impressive.
Of course when you are filling a 35-50 gallon tank at $2.00+ per gallon, that's probably small consolation.
Have fun with the bus!