IMO that looks like Galvanic reaction from dissimilar metals.
" Galvanic corrosion, also known as bimetallic corrosion, is an electrochemical process whereby one metal corrodes in preference to another metal that it is in contact with through an electrolyte. Galvanic corrosion occurs when two dissimilar metals are immersed in a conductive solution and are electrically connected."
This has been an issue in the likes of automotive in the past such as back in the late 70's when the aluminum bumpers were fall off of GM cars while driving down the road….. if you old enough to remember that deal.
More recently Gen 1V Ford Explorer tailgates come to mind ( as seen here )
those are only two examples that I can remember right off the top pf my head.
Maybe Im wrong but that is certainly what that looks like and if so?.... I highly doubt it can be stopped …slowed down maybe… but my guess is that issue goes way deeper into the coach than what is exposed in those pictures.
I hope Im wrong for the OP sake and others with that issue …… but I’ll take “ Galvanic reaction for $ 20.00 Alex.
IIRC from 7th grade science class that this reaction can be generated from something as simple as using aluminum rivets in mild steel or the likes of ..or visa versa.