Is the trailer a twin or a double?
If its a double you're missing the upper part of the slides that fit into the "U" channels. The front of the bed frame, about 4 inches high, attaches to the slides. Plywood covers the frame.
The framing looks a bit different from my TW but the principal is the same. Lift the front of the frame off the stops, pull it out and voila! a bed. Well, OK, something you can sleep on with a crease in the middle that's a pain to put sheets on.
You get the idea.
I don't know if the OEM parts are still available.
Wooden slides are available if you are inclined to rebuild the bed from scratch.