It really all comes down to configuration. As I recall the rear seats (4 across) are not removable, and none of the seats fold down in the Van. I have read that some users were able to get a couple of the seats along the passenger side removed and stored at the rental location, but I'm not sure if that is an option or if there is a charge.
Though a bit awkward I would think you could get some luggage in the back seat. There is very little storage in a 15 passenger van. If you can get a 12 passenger on 15 passenger wheelbase there would be a lot of storage.