RV Repair: soft wall in bath, spray nozzle, fiberglass resin

Question
We noticed a place on the rear wall of our camper (1987 Marathon TT 25 ft) that had black mold where the seam trim is on the vinyl coated wall board. This is on the back wall about 2 ft above the tub. Upon pushing on the wall it is soft in this area and is near the back curve of the wall (angled). We are not sure what to put in it's place to repair. Is this wallpapered board water safe? I was thinking a fiberglass or plastic flexible wall would be better, but don't know how to find one for odd curved area around bath. There is no other rotting occuring at roof...only thing that penetrates camper near here is top light on back or maybe rear roof seal, but ceiling isn't rotting. Need help on what to replace it with/how/and if you think rot is from water from wall mounted spray nozzle in bath or coming from exterior. Thanks in advance :)

Answer
Frp or  fiberglass resin board is flexible enough to use also tin flat pannels for a shower surroun are also good. The rot is  prob comming from the shower but wichout pic I cannot tell you
most of the material listed above you can get at a lumber yard be sure to seal it good




Thanks and good luck