The exterior wall in the bathroom, behind the fiberglass bathroom shell was in good shape, I did cut in a small window in there, if needed you could do that to access that wall, also I removed the medicine cabinet to access the wall too. Under the sink and if needed if you pulled the hot water heater both those would offer some access to that wall.
I did do some rebonding of the exterior wall behind the heater and fridge
, emptied that area then rebuilt the wall.
In inaccessible areas that have dis-bonded try to inject epoxy between the layers using a large syringe with thin wall flexible tubing attached from any accessble edges, or if necessary inject the disbonded area by drilling a small hole thru the interior paneling, inject with epoxy, plug the hole and clamp or block the area flat till the epoxy cures.
Btw, the fiberglass bathroom pan was not bonded to that exterior wall from the factory, so it is normal for it to have some flex, behind the pan is the same construction as the other walls in the camper, exterior wall glued to the foam core and wood paneling glued to the core on the inside.
That's where the dis-bonding can occour.