Do the portables have a condensate drain? Is there any reason you would need to regularly access to the back of the machine? I'm thinking about putting a dual-hose portable directly against the wall (or as close to that as possible), using two threaded fittings to make it water-tight while driving.
I'm planning a rear galley, with only the side door to enter/exit the sleeping area, so a window shaker would require cutting a gigantic hole in the side, finding an RV baggage door exactly the right size, etc.