Have you thought about an "add a room"? (
far too pricey for us to buy one... but I could sew one up)
Could go a couple of ways on this. you said you had bunks... could you add a platform/deck out the back (removable stabilized receiver mounted thing like a big hitchhaul), build a PVC frame, sew up a canvas cover (like painters drop cloths) to attach to the bus once parked ?
Perhaps do the same on the side of the bus which would also allow you more "living" space so things aren't as cramped with more inside. Use heavy duty velcro and keep the thing simple. You know folks USED to build their own tents. I'm sure there are patterns/plans on the internet somewhere.
PDF Tent Pattern (Free)
from the forums I found it on... read down, interesting reading... "I spent a few hours planning out a large wall tent that can be made with painter's tarps from Lowes/Home Depot, etc. Being the picky person that I am, I made a 1/2 scale pattern. It's currently still being sewn, so the end results are not conclusive yet. At any rate, I thought i would share my pattern with everyone. Enjoy!"
Another idea might be a modification of the bed ends that were on our Apache popup.
The beds were simply a plywood deck with ABS panels. It all folded up flat (side walls folded down to lay on the bed then roof folded flat to rear wall which then laid down on top of the side panels). BTW, a "living hinge" is a fairly simple thing to build. We had to replace ours. Ordinary RV awning rail (one on each side) with a length of heat cable
(the kind you make a heated water hose from..comes in various lengths) acting as a "pin". Ours lasted for a decade or so. We did have to crimp the awning rail a little (used a vice grip so we had the same clamp for each squeezing... very easy to squeeze that aluminum rail too much). (click on image to see the whole thing. Living hinges don't leak because they have no open knuckles.