Check out this post for one potential idea - it should work at least until your little one gets mobile. The hardest time is from when they're first mobile (able to bail out!) until they're old enough to get down safely on their own...
Your floorplan looks to small, but we were always able to use pack and plays. We had twins just before getting our first trailer. The first trailer had jack and jill bunks (twin over full). We had the pack and plays in lower bunk (mattress removed) sideways so they could stand up and we had access to pick them up on the open end away from the top bunk. On the new trailer, we have quad bunks in the back. The pack and plays sit sideways and end to end on a platform that goes between the two lower bunks.
Starting this season, I will be making the rail in the link above and the twins will share the larger bottom bunk. The trailer now feels twice as big using the bunk area as intended!