That is one of those questions that can be hard to answer. It can vary with the way one drives and the condition of the vehicle your driving and the weight you are carrying. You can begin as low as 4 mpg and end as high as 12.
Do u have an allegro bus???
I own one it,, is about the same year as u'rs and i average 11 mpg if i am not towing my other car.. but,,, (get this)..
If i tow a car it gets 12 & 1/2 (go figure)
U have an engine brake on u'rs ,, also my has the 855 cummins in it....