I have a 76 ARGOSY with that mid ship bed design.
These are photos of Grant, friend and fellow member DavidsonOverlander, installing the slide out drawers that he built for me and that go under the bed.
You can see the original aluminum frame, supported by the drawer holder, and figure out how it works: a sheet of plywood, with wooden runners attached, lays on top. The sheet pulls out, mattress/cushion on top, with the runners inside the metal channels.
The cushion is a very simple folding mattress that any good upholsterer can do. You can do it too if you are handy and have sewing skills.