Her comment about having a place to sleep could have meant nothing more than she'll need a place to sleep...kind of like saying if I have food on my plate, a place to sleep, and a hot shower I'm a happy camper!
Still, it's best to be clear about the sleeping arrangements and really all you have to do is let her know what hotel you're staying at so she can get herself a room there too. Letting her know that you and your husband prefer your own room is very okay, as other experienced swingers have said already, it's standard protocol for them for the reasons they've mentioned. We always get our own room when we meet couples.
I play solo with couples where I stay with them in their room because they invited me AND - most important - I've played with them on many occasions as a couple, with my husband. We knew each other well and had already played so we all felt very comfortable sharing a room. Even then, if I would ever play with a couple who I'd played with before, and they preferred separate rooms, I'd be fine with that. I may even prefer my own room with couples who I know well and I'd not hesitate to let them know I'd prefer my own room. I make that decision based on the couple.
I see no reason for you to pay for her room. Any single should be willing to pay for her swinging costs, just as a couple does. It's nice when the couple pays for my drinks or dinner and they most always do, even though I offer to pay for myself. Sometimes I pay for their drinks, it's a give and take with people you continue to play with. I also always offer to pay for half the room cost when I stay with a couple.
At this point you don't know for sure you all will play. All the more reason to have your own rooms.
On a final note in response to another post: How much you think a person makes for a living should never be a consideration when it comes to paying your own way. If I was single I'd find it offensive if a couple presumed I didn't make as much as they do just because I was single.
LM