Yes, I did build a full time bed that is clear underneath full width for storage. It is documented on another forum but it may not be there anymore. If you search my posts, you could find various pix of it in other discussions, mostly in the power system change ones that show the rear a lot.
It is a simple 1" square tubing welded from with 3/4" light plywood on it. It is has a hatch door in the front for access from there if needed. We got a 5" latex mattress for it custom made for good fit. We would never, ever, go back.
