Despite my first comment, I do feel your pain. Not just from the present condition but from the devious way it was sold to you and yes, if it's that soft, it's rotten and water logged underneath and fixing the outside, does nothing for the inside and yes, it's a major job that you may be up to but make no bones about it, it's involved and time consuming and the mold that is growing inside the walls is very unhealthy for you.
patching the outside does nothing for the inside and will not remediate what is occuring under the outer skin.
My last unit also had issues and I sold it for a loss and I was honest with the buyer as well. I went on to a new unit and believe me, I watch it like a hawk.