The full-size bed will be located at the forward end of the trailer, across the width. (This configuration allows enough room for wheelchair approach.) We screwed 4' lengths of e-track to the walls, using self-tapping metal screws where we could anchor to one of the vertical steel studs. 2" x 4" lumber was cut to length and attached using 2x4 sockets.
The completed frame shown above is only 4' wide, but a full-size bed is 54". We added supports to the side near the wall with 6" pieces of 2x4 (not shown here, I'll try to get a pic when I can.) I don't plan on sleeping that close to the wall...it was mostly to keep the mattress from sagging.