There are those tire covers that you can get. I just did a google search for "Bus Tire Covers" and the very first one mentions a size cover for buses, but doesnt seem to carry them...
I have seen people use plastic, drywall, plywood, and my favorite, folding metal.
The metal one but a cutout that was just big enough to fit into the wheel well, but had arms that folded out at the bottom for better support. I think it had latches attached to it and the body of the vehicle to latch it in place on top, but I cant be sure as I only saw it once and that was several years ago. Paint it to match your bus...
I have an idea for something simular, but it would involve having a metal box mounted to an old wheel hub. When you park in one location for awhile, you take the tires off and put these on. They wont care about the sun, but they might rust.