Sandy, the back bed has a piece of 5/8" plywood instead of a table between the white seats.
For the rear mattress, I covered the foam with a mattress pad and the fabric, turned it upside down, placed a piece of thin 1/4" plywood on top, and stapled the pad and fabric to the plywood. I only needed thin plywood on the bottom of that mattress since it was not going to support any weight
I also used 1/4" plywood underneath the small dinette bed, but that was a mistake. I had planned to secure a 1" x 4" board with brackets to the outside edge of the dinette cabinet to support the edge of the mattress. But the water heater switch made it impossible to fit a bracket at that end of the bed.
I tried using the 1/4" plywood base with nothing supporting the edge. But when I sat on the bed, the plywood flexed and pulled the staples out. So I ended up screwing an additional piece of 1/4" plywood on top of the original one.
It works, but if I were doing it again I would have used 5/8" plywood for the dinette mattress since it extends over the edge of the fiberglass cabinet.
The new dinette cushions keep the rear mattress from shifting. I use a piece of the rubber waffle-weave non-skid shelf liner from Walmart taped to the back of the plywood on the small dinette bed to keep it from shifting. It's probably not necessary, but it also helps protect the fiberglass base from getting scratched by the screw heads.
I still keep the small dinette table in its place under the bed, but it does slide out partway when we are in transit. If I had used 5/8" plywood as a mattress base, no support would be needed under there.
I learned a lot in that project, and someday WAY down the line, I might redo it and fix the mistakes I made.
Hope that helps.