Hi,
We just came back from Boardwalk Villa, similar situation like yours. It was me, DH, my 18 months old and my 4 year old, plus my mother. We had a two bedroom, it was great. My mother loved her own privacy in the 2nd bedroom, plus the 2nd bathroom.
However, since the kids slept with me on the king bed in the master bedroom, DH slept in the living room. I was really glad that we had a 2-bedroom, otherwise, it would be a little difficult in terms of the sleeping arrangement.
I think you can manage it, but it would not be as comfortable as a two bedroom of course.
Oops, I just re-read your post, you have a 18 year old, not a 18-month old, sorry, I misread it.