24T to 24D conversion
Here's what I did. Original to the trailer was the twin bed set-up, one twin over each wheel hub. I took one twin bed out and built a very large drawer on top of the base of the other twin bed. I pull the drawer out to extend the bed into a full size. It's actually 6" less wide than a full size mattress, easily purchased anywhere. I used a memory foam mattress.
When the drawer is in, it locks with eye-bolts and doubles as a couch. There are plywood pieces underneath the mattresses for stability of course. The fabrics to cover the mattress is pending.
Good luck with your choices!