Orion, I had a similar situation with our master bath in our last house. Upon removing all the fixtures including the bath, I found the floor under the corner shower to be rotted beyond repair. (it was a 60's house). I ended up cutting away everything up to the bottom plate for the walls, installing sisters and then replacing the plywood and then adding wonderboard. I had the luxury of an open work area, but I understand your frustration.
RJ, One thing my wife and I decided after we 'discovered' BPlus type rv's was that we did not want to have the same bath compromise Class B owners deal with. Turns out it was a great decision.
Gene...you are the best! :)