softness is caused by give in the white foam part of the sandwich.. commonly called delamination.. fixing it has been well written up..
white foam was never meant to be walked on so its not surprising it gives after a time..
having said that a rotten plywood floor is a different thing.. you need to find out if the ply part of your floor is rotten or its just give in the foam.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.