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.
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets