There are so many variables to MPG that it is impossible to say what any given bus will get. However, general rule would be that if its Gas, less than 10 MPG (6-10). If is a diesel, and depending on what diesel, you can get 10-15. Stay away from 6.0l Diesel Fords, they can be good, but not worth what it takes to get them there unless you are a mechanic that knows 6.0L Fords. But because you asked, I assume you don't.
A GM bus would be gas or could have the 6.5L Diesel in 98 (I could be wrong) and I have never seen a Duromax in a van. The 6.5L or the gas engine would be OK, but neither were that good on fuel, nor were they good for much over 200K.