Personally, I think you are better off keeping with the faux canvas paint job. But if you are determined to put something that looks like real canvas on the outside I can think of a couple of ways to go.
First way would be to sculpt sections out of fiberglass and screw them on to the roof. You could even make a mold. Lots of information of sculpting and molding fibrglass on the internet.
Upside of this approach would be that you get a smooth surface that should be easy to clean, it's also light and you could make the roof look puffier. You could even create a void between the bus roof and the fiberglass roof that could be filled with spray foam insulation.
Another way would be to embed some type of cloth, maybe burlap, in many, many layers of elastomeric roof coating. I think you could put down a thick coat and then while it was still wet, embed the cloth into it to get a draped looked. Once that is dry, put like 6 coats over the cloth with a brush making sure the cloth is completely covered. I think this would work but i would question it's long term durablity.
Scratch what I said about burlap, you could use a synthetic shade cloth and embed that is elastomeric.http://www.gemplers.com/shop/shade-cloth-b