Sleeping options with a older baby

I'd just use a pack and play. You'll have to put it up every night and take it down every day to optimize your interior space, but you're right in that a 9 month old is too young for sleeping on a bunk bed. Next summer, you can put her on the bunk and just use a bed rail to keep her from falling off.
I remember my daughter sleeping on the bunk at 18 months. Her bunk had a window and when I'd put her down for a nap, she'd play peek-a-boo at us with her window shade, lol.

We use our pack-n-play for our 1 year old. We lower the table and put the pack-n-play on the table between the cushions. In your situation, I would try to have you and the wife sleep on the bunk or the sofa and the kiddo in a pack-n-play.

By camping season 2017 we plan to have our daughter in the top bunk, she will be 4 and our son in the bottom bunk, he will be 2. In the meantime we don't use our dinette for eating... we eat all our meals outside.

bunk mattress would be fine if you had a bed rail like this:

http://www.amazon.com/Regalo-Hide-Away-Extra-White/dp/B000H1IYXW/ref=sr_1_11_s_it?s=baby-products&ie=UTF8&qid=1454515907&sr=1-11&keywords=bunk+bed+rail

Our friends used something they bought at a baby store. It was a rectangle made out of round foam covered in cloth. The toddler lay in the center and the round foam kept her from rolling out of bed.

Something like this but toddler sized
http://www.plioz.com/lulyboo-baby-lounger-to-go-baby-travel-bed-with-waterproof-bottom/

My son was older but I debated buying an Ikea crib and customizing it for a bunkspace. We ultimately didn't, but that was the thought.

Before that, we used a pack and play. It was big and annoying, but it worked. We had a over 2x more space than you, though.