I'm adding a 2nd bunk in ours. The hardware for the original is fairly straightforward. If you have some fabrication skills, it wouldn't be too bad to make something similar.
I noticed in the Trading Post that another member has some interior parts for sale--it may include the bunk hardware:
The difficult part if yours never had a bunk may be providing the support behind the walls. I don't know if they put the bracket in place if it wasn't getting the bunk. If it's not there, you'll have to figure out how to provide the proper support. I don't think I'd depend on the wallboard.
On mine, the upper cabinets in the front of the camper have a "false wall" inside, toward the sides of the camper. You can remove this to access the mounting location for the bunk hardware, as well as some of the wiring that is in the front cap. Might be worth checking if yours has this, and if you can tell if the brackets are there.
Not sure if all of that was clear... Mine is all torn apart right now, so I could provide some pictures of the structure, but not of how you're supposed to access it.