2004 T139 Fun finder shadow cruiser shower walls

[rj4hunting]

Hello, I just bought a 2004 T139 fun finder and I have a smell I can not figure out what it is, I think I know where it is coming from which is on the outside shower wall, I pulled the fridge out and can smell it on the plywood but only on the shower side. it is also a darker color than the other side,is it possible the factory coated it with something, what does formaldehyde smell like, I need help on this, Thanks