They have non rooftop mounted units that are for campers and semis. It would take up a little space somewhere inside or under your bed but they aren't real big. I looked at one for my burb quite a while ago. They would normally mount in one of the small tool storage areas under the bunk in semis or one of the side door compartments on a camper....
Have you checked out Red Dot? They do a lot of stuff for semis and other custom applications. Here's the catalog link http://cld.bz/abyYMy#62
I bet they have a smaller unit that would work but I am not sure if their units all have to have the engine running to run a compressor. Google search some 12volt a/c units, one that comes to mind when I used to work on semis is Webasto. They do 12 volt a/c units and also diesel fired 12 volt heaters. Run something like that and maybe a small solar panel to keep the batteries charged just like in a camp trailer.

Would it fit mounted to the back of the cab below the rear window on either side of the gas tank?
Why don't you dd it again?

props for plowing thru all the drivel.. 
Dont really get down that way much, but if you ever need a hand I'd be happy to make the trip down to help out if need be.