Wheels, I actually have some decent experience with this. I have a 0* synthetic North Face bag that I keep compressed when stored (lets say 11 months out of the year, conservatively). I havent experienced any noticeable drop in performance. The deal with synthetics being compressed for too long is that the insulation material can crimp. However, if you remain with the van, or in another sheltered area, it wouldnt be noticeable. And, it takes years for the fibers to "remember" the crimping. Overall, in my opinion, it isnt something to worry about. Down is a different issue; you spend easily twice as much on a quality down bag, and every little thing you can do to extend its life is worth it.

I would recommend a Kelty synthetic bag. They are inexpensive, and work quite well. Hope this info helps!!