I saw one for sale a while ago online.
The owners took out some front cabinet work and one of those tables that stick out from the wall (that has the free standing chairs) and had a triple bunk installed (bottom bunk on the floor). It was tight, but looked really good -- like it was purchased that way. Would be too short for most adults or bigger kids (as most of the standard bunks can be).
Depending on your budget, I would do a search on some RV remodelers in your area just to see what could be done, cost and so on. If you're handy, you just might be able to remove, say, a booth dinette and install something on your own.
For temporary solutions:
We use inflatable, residential height aerobeds in the aisle (great for bigger kids/adults). Super easy to set up and also to store.
Another thing to try, although you'll have to set up/take down which can be a pain, are camping bunks. Cabelas has some here