Airstream has gone through several refrigerator venting systems. Common on the mid 60's to sometime in the late 70's or early 80s, the air came in through a cutout in the bellypan and up through the floor behind the refrigerator, past the frig, and out through a plastic refrigerator vent shaft to the outside vent on the roof. Worked like a charm.
Then they eliminated the underbelly vent and floor vent and had the air enter through a perforated vent which also served as the refrigerator access port, low, near the floor. Same exit, up through the roof. Worked OK, but not as well as the floor vent system.
Recently they have been using a side vent low, and a side vent higher, with a fan to assist the movement of air. Reports have not been very good on this system, with poor cooling of the box, due to poor heat elimination. It also uses power to move the air, not good in boondocking situations.
So, yes, you probably could side vent the refrigerator, high and low the way the current units are set up. I don't think it is a good system though. I soon will have my 2014 with hi/low venting, and will have better first hand experiences in a month or so.
Hard to beat gravity air flow with an almost 8' height difference top to bottom.