Both rows of seats fold flat into the floor on Stow-n-Go minivans, so you're good there. Town and Country would be included here, too. These would definitely be your best bet.

All the other minivans have removable seats in the 2nd row, but occasionally rental companies secure those in place so they can't be removed. I can't comment on whether National does that universally, but you'd want to check before you left the lot if you rented something other than one of the Chryslers.