Your construction experience will, for sure come in handy.
I will try and explain the complexity of the shower. Campers are not built like a house. They are built from the outside in. As such, they do not repair well taking them apart from the inside like a house.
If you want to replace the floor under the shower, I recommend to "not" to take the shower surround out or the shower basin. The surround walls are glued to the camper walls that are 1/8Ē thick luan. The damage you will cause to the surround panels trying to get the wall panels off and not destroy the wallboard or the panel has high odds you may have many issues to fix after. And the shower does not have to come out to replace the floor as the shower basin is approx 4" above the floor line.
The next part of the issue is, the camper floor is "under" the walls. The walls sit on top of the floor. While you can remove the old floor with the walls in place, adding a new floor can be an issue as there is no wood to screw it too on the perimeter. And then there is the reality, odds are very high the outer band board on the back wall, and potentially the rear left side is rotted out. Floor joists most likely also rotted. The OSB floor didnít go bad just by iself. If the outer band board is gone, soon the walls will start to compress as there is no wood to hold up the wall studs.
Here is one approach on to correct the floor, consider this a guide to help you think through how you will do it.
First, use the moisture meter to confirm there is no left corner leak rotting out the back wall along with part of the left rear wall, both which are behind the shower and check up in the ceiling, at the rafters. Odds are high a leak can exist in corner molding and the rear left roof corner joint in addition to the dented siding. If you post pictures of the roof caulk in that left rear corner, amd some of the corner molding putty tape sealant, we can see how good or not this is to help better. The shower dome can also be leaking. Many times you cannot see leaks inside the camper living space, vinyl wallpaper allows the water to stay behind the wall that is soaked.
Since there was siding damage in the back, and you feel that was where the water got it, the water amount was large enough to rot out the floor. The floor is on top of 2 x 3 floor joists which are sitting on the metal frame. It takes a lot of water to rot the floor board that high up. Odds are high the lower part of the rear wall is rotted. Meaning the bottom of the wall studs and the bottom band board needs to be replaced. Again, 30 minutes with the moisture meter and you know from the start of what you are up against.
Once the rear wall and bottom of the camper are confirmed wet, I would take the rear wall siding off. If the rot is low, only the bottom 2 panels may need to come off. The LH and RH corner comes off too which odds are the putty tape sealant needs replacing.
With the back siding panles off, you can look at the back of the camper, see the floor, under the shower and the band board. The rot will then guide you how much more siding needs to come off. Odds are high, the wet has moved under the bed too to some degree. You can take up the bed and the bed compartment without too much issue to repair that area.
Then cut into the floor where you can make a good repair joint and if you are "lucky", it is only the left rear corner that is bad. Point is, you pull out the floorboard from under the shower basin. The shower stall stays in place. There may be a few floorboard screws to deal with if they are not rusted off, but the sawzall cuts them too.
How far the rot goes out into the sink area is still TBD. The meter will tell you if it is wet and how much. And it may be partly wet that can dried out and be treated with resin to leave it in place. The bad wood needs to come out and be replaced.
You may have not made it to the moisture meter link I talked about in the last reply, there was a link to this post which shows how the camper is built and how it can rot. The older Sunlines has floor joists that ran front to back, then they switched left to right. Iím not sure what year they switched. That could be a difference.
When you have the back lower siding panels off you can trim through the lower 1/8" wall board and get right into under the shower. See here. The shower stays in place to service the floor.
You can see the shower basin Styrofoam base several inches above the main floor that you need to replace.
Hope this helps. Ask away if I have buried you in details. Glad to help.