Let me try this again...
Here is what I found under the vanity. Just soft wet mush.

This is what is left of the center stringer. You can also see that the cross support that should run under the rear wall is gone. It just crumbled in my hands as I pulled it out with my fingers. No strength left in it.

After all this I could see I had to pull the tub. So this is what was left after removing the soaking wet insulation (literally had water dripping from it as I removed it) and shop vac the floor.

There is nothing left of the stringers, subfloor, or outer boards under the walls anywhere that I have opened up so far.
I think I am screwed...