I've been dealing with this issue for years. Sometimes after a night's sleep my back is barking at me in the morning. One day I recalled how many many years ago I used to sleep on the floor and how that helped a back issue I was having at the time. I won't go into the long story behind that but it made me think of a possible solution in the RV. I'd place a piece of plywood on the bed.
Next stop, ACE Hardware.
Here is the bed naked. The right half that Beans is on lifts up for access to a storage area at the bottom of the photo and the heater compartment under Beans. I used to have two inch and a half memory foam mattress toppers on there so I wouldn't feel the crack. I eliminated one of them while at the house back in May. That helped.
This is a four foot by four foot sheet of plywood. Anything longer the corner of the board would stick out along that curve of the mattress in the lower right.
I really didn't need the stiffness for my legs anyway.
I added this heavy wool blanket for the little bit of padding and insulation plus it was in the way up front where I stored it. Here was a chance to use it and get it out of sight.
Then the remaining inch and a half thick memory foam top covered with a comforter.
Finally the sleeping bag and Mexican blanket.
This was a huge improvement. The first morning I sat up to watch the sun rise and it dawned on me (no pun here) I was sitting up! I never could sit up before as it was all too soft, no support.
Yeah Dad, I like this.
I do too Beans.