First off, welcome!
Bummer on the water damage. I have a 2004 T1950 all apart and in the process of restoring. I am not sure how far you made it taking the camper apart yet, but try and not take the wall board out from the inside if you can help it. It is easier to repair the wall board and the rot damage with the siding off from the outside. If the wall board is already gone, we can help with that too.
See this post which may help. It will show more damage (hopefully) then yours and how it all comes apart. You can read all of it or just skim to the parts that are closer to yours for fixing. There may be small difference between the 2000 and the 2004 model, but the campers are built very similar.
Removing the shower surround itself, is a major undertaking. You can cut the rear wall, wallboard "under" the shower basin with the rear siding off to to expose the floor. Pics are in that post above. The shower basin "might" come out, but I have never tried doing it without the taking the surround out first.
If you can get to the screws holding the basin support wood out, which is a hurdle it and in itself, then the basin may be able to drop down from the surround and come out the back wall totally exposing the floor. The hope is, you do not have to remove the basin and deal with the floor under it with the basin in place. This sort of all depends on how far the floor rot went.
This should get you started, post pics of the progress and any questions you have so we can see what you are up against.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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