I know that these can be cheap if its a wally world window unit, but check craigs list for ac's and just add a roof one for the rear. Ive seen roof units for a couple of hundred advertised. Just would need someone to run a circuit to it and you see these being shown in the rv magazines a lot.
Just a thought.
Just took a look at CL here locally: $500 about what a 9000 room (like the above picture)air costs and all its associated hassles.
http://phoenix.craigslist.org/evl/rvs/4537668062.html
http://tucson.craigslist.org/rvd/4539036179.html
