Regular resorts have some rollaways but they are not allowed in/provided at all at DVC resorts (rooms). If you want more beds then provided you are expected to bring your own including the linens.
Not a rollaway, but I rented a toddler bed once and didn't have any problems. I second the air mattress idea, though. And I'm sure you could even get them to bring you an extra set of linens if you didn't want to bring them. They've never been stingy with them.