If you want a stone interior, the model railroad crowd sculpts and paints amazingly realistic stone out of xps foam. They are doing it in scale, making cliffs and mountains, but it would be much easier for you to do it life size, making one thin face of stone (or brick, really) at a time.
It isn't very durable, but its light, easy and cheap to make, and easy to repair. Would be perfect for the ceiling and walls above the waistline. Compared to paint, thin blocks of foam stone would be stunning, and help quiet and insulate the bus.
There are many videos on this. Here's one to get you started: