The first thing you need to do is add something like stick on floor tiles, it will allow the mattress to slide around.
//heartlandowners.org/showthread.php/11365-And-so-it-begins?p=123816&viewfull=1#post123816
Here is the start of what I did.
//heartlandowners.org/showthread.php/11365-And-so-it-begins?p=186203&viewfull=1#post186203
When I finally got around to finishing this, it was because I had left something under the bed on top of the rails that fell off while traveling.
When I opened the slide the item jammed between the floor and the bed framing. That ripped the frame apart and required some repairs. That is why there are those 1' x 1' gusset plates screwed in.
The new storage section is only about 4" deep, but it is better then the wasted space.
The struts I am using are not strong enough to hold up the bed (and I think they are on the wrong angle), so there is a plywood pocket you can just see on the one picture (at the foot of the bed) and a cleat on the mattress platform (same picture near the top of photo). There is alos a 1/2" strip of plywood glued to one of birch pieces so that that both sections sit level.
What is hard to see in the photos; I used 3/4" plywood set inside the 2x3 framing and glued to the outer 1/4" finished panels. These are cut so that the 1/4" birch plywood rests on top of the steel frames for the slide out and the plywood cleats.
Since I had to cut the 1/4" birch section in half just to get it inside the frame, I also added the cleat you see at the foot of the bed. That is also the pocket for the wood dowel (old flagpole actually)
If you try this I would suggest removing the mattress (out of the bedroom) and after you remove the mattress support panels have someone operate the bedroom slide so you can see how it works.
I have thought about cutting into the panel that would be facing the front of the trailer and boxing in that void behind it for a hidden storage compartment (firearm etc).
But now that it is back together I just don't feel like taking it all part again. At one point I did have the whole bed frame in my driveway re-building it from my oops.