Have you checked with a local propane wholesaler or servicing dealer? They usually have catalogs from more than one supplier, targeting a wider range of applications than RV installation.
I think there is a relatively long 10 inch tank that is close to the capacity of the short fat 14" diameter 40# tank used on a lot of C's. I've been looking where to put a tank on my regular length E-350, having a hard time on space too. I don't know that we can get around the need for an access door. If you install it so someone has to crawl under the van to fill it, could have a hard time getting anyone to do that.