It would appear that play pens have made numerous advances in the 30 yrs since our daughter was 6 months old. But we had a play pen that unfolded very similar to the way camp chairs do. Opened into a square similar to this:
Graco Pack 'n Play Play Yard TotBloc - Harris | BabiesRUs
did not have legs, the bottom sat right on the floor. We put this up on one of the bunks of our popup. She slept in it for 2 years, then her brother slept in it for 2 years. Worked very well.
You might like something like this a bit better:
KidCo PeaPod Travel Bed - Red | BabiesRUs
Note that in our popup, the back bunk was only 10 ft away and the view between us and the bunk was wide open.
I'm guessing you're the DH, not the DW. I strongly suspect the DW is NOT going to want to put the 1 yr old in the back bunk, 20 feet away, with only a few feet of opening to monitor what's going on. I strongly suspect you're going to end up losing that floor space at night.