RV Salvage yards have rooftop AC's that can be had for cheap compared to a new one. The rooftop AC will cool your bus somewhat but it doesn't help much driving down the road, as least not in my old motorhome, wife and grandkids in back bedroom watching a movie on TV and freezing while I'm burning up up front driving the coach. If I ever get a bus the front AC will be over the driver's compartment
with the blower pointed on me.......
Of course you'll need a generator to run it.