My new trailer has a MaxxAir with the integrated cover. It tilts up when you open it. It's a very cool fan and quiet too. It has a much lower profile than a standard vent with an added cover - I'd say it's at least 3 to 4" lower (when closed).
Assume you are talking about one of these:
http://www.amazon.com/Maxxair-0007000K-MaxxFan-Deluxe-Remote/dp/B003ZOF09Y/ref=sr_1_2?ie=UTF8&qid=1425087007&sr=8-2&keywords=maxxairI see no reason this couldn't be fitted to a B, as the opening is the same size as the fantastik fan. It would be a major project to tear out and put a new one in I suppose. But if you are having a new van built, I see no reason not to spec one out.