When we got the camper, we always knew that the inside of the bathroom wall was damp.
There was a cupboard hanging on for dear life, against the only bit of wall that hadn’t been clad in 1/2″ thick soffit cladding.
Where the mirror and sink and internal wall is, the wall there had had water come in from above around the heater flue, so that has been fixed. The problem, like so much of the rest of the insides, is that rather than fix the issues, everything literally was covered over, either in cladding, pine boards (and 1 1/2 inch screws) or laminate flooring (on the ceiling!).
So the plan is to replace the cladding in the bathroom with 2mm PVC Sheet walling. Removing the old cladding, we can see that the water has been getting in through the rear gutter screw holes, the rear awning bracket screw holes and the join between the rear panel and the NS Panel, through which you can see daylight!
So, the plan is to remove and reseal the gutter and awning bracket. Brace the two panels, replace the polystyrene with compressed foam and then finish with the PVC sheeting.