I've thought about this a lot. In the "Campmore" catalog, there is a Eureka
tent which has a front AND a back entrance. Right there you're half way home.
The tunnel is a short tarp held in the raingutter with a 1/2" diameter wooden
dowel rod and a couple of clamps (the ones that look kinda like cloths pins).
Mosquito netting sides with magnets to hold to bus, sew a few buttons onto the
tent, button slits sewn to the mosquito netting.