Okay, I think I've got it! I found a foam mattress that is 25" wide at
Amazon. I'm thinking it can fit on the floor at the end of a bed (possibly can even be partially under the pull-down bed? I mean just a couple inches or so of the mattress, not enough so that the person on the floor would hit her head or anything like that), just while we sleep. We will need 2 rooms and hopefully will get connecting rooms. I'm confident we can figure something out one way or another. If it absolutely won't fit, the girls will just have to tough it out sharing a bed. During the day we can either store it under or on top of the fixed queen bed.
Edit: I'll probably have it shipped from Amazon. I know there's a fee (still $5?).