I'm not sure I'd want a new sidewall in this situation; rather, I'd seal the backside (the side facing inside, towards refer, where you won't see it) of the exterior vents with something that would last indefinetely for you but be removable by the next owner in case they for some reason wanted an absorption refer installed. This option would be easy if all gas and electric needed was already in place. I don't know what the backside of PC refer vents look like, but if anything like the lower metal louvered hinged door on my father's '97 Roadtrek 170, it shouldn't be too difficult to remove it and fashion a lightweight sheetmetal cover using tinsnips to cut something you could then bend into a cover that would look like a metal cookie sheet. Silicone caulk would seal gaps and adhere the sheetmetal to the backside of the louvered door. Additional weather stripping would seal the door itself tighter than before.
Of course, if PC vents are different your design will vary.
I'd probably go the above route rather than risk leaks or worse after replacing an otherwise perfectly good sidewall, with all the work that would entail.
Another thought: I've often wondered why one couldn't have a damper that gave a compression refer user the choice between inside and outside ventilation, based on whichever was cooler. No point in running your heater in winter, and then using more energy for heat exchange going the other direction to chill a small box for food when cool air is right outside. You might have that option, now, but the design of it is beyond me, without actually seeing it. Hmmmm....
