Well, a soft floor obviously means a leak occurred. This probably happened over the 9 years of so of ownership depending on your storage, maintenance, and other rv related habits...
Since you are finding the soft floor now, all you can do is start tearing into it bit by bit until you get to a point where it's not soft anymore.. How much, what it takes to fix is up to what you find..
To pay someone to do that will be a very $$ job... So you will have to weight the value of a 9 year old rig to what it'll cost you to have it fixed.. (either yourself, or pay someone else).
If you are a DIY person and like the rig, then it might be worth your time to check things out.. Or, just unload the thing and move forward...
Good luck! Mitch