Your only option as far as I'm concerned would be something like this.
You'll have to go to smaller tires and most likely raise it as well. With the smaller tires you're going to want a different gear set in the rear. So by the time you do all the modifications and spend a bunch of money, you're better off just buying what you want from the get go. THEY ARE OUT THERE, just take the time and look far and wide.
Actually, now that I think of it, someone on here has this exact style bus. If I could only remember their name