[cwl1979]

Sounds like you've got a rear bath with shower in the back left corner like I do in 435. Just curious when was the last time you were up on the roof and checked the sealant for the end cap or bath fan? Could have water penetration coming in through there and running down the back of the shower walls and sitting on the floors under it. Does the shower pan feel soft?
Other possible causes could be the sealant from the shower walls and corners has failed and needs to be fixed.
Either way with the structure under that shower area being that bad it might be a full bath remodel to repair the floor underneath.