Not familiar with the mattresses on that model. We had a ROO 23SS and it also had the straps to hold the mattress against the deck when folded up. You could try something like a strapping and add a sturdy snap to at least hold both sides together. I always wished for some tidy way for the tenting to fold up instead of just hanging down, but never did. Moved to a TT last year, so it's a moot point for us now. Hope you can work this out.
BTW....do you have a mattress topper you're trying to fold in as well? That could be a problem if so. Take the topper out and then try.