Holy cow, Just looked at your new pics, that looks like you are working on mine, just about the same rot in the same places. As for removing lil johnny, mine was so rotted it really wasnt much trouble getting it loose inside the camper. The floor wa completely gone in mine, so the only thing I had to do was separate the stall from the black tank below it. a saws all worked well on that, and since I wasnt going to reinstall the stall, I cut it in half to remove it from the camper.
btw, brace that area good, that shower stall is more support than it looks.