[JRTJH]

Lots more information would be needed before any advice could be provided. First, how is the unit stored? How often is "roof inspection/maintenance" performed? Have you noticed an wet/damp walls, floors, window attachments? Any issues with wet carpet around the slides? How is the unit winterized? Have you checked for plumbing leaks? How extensive is the "wet area under the sink?

That's just a start, but with more history/photos/information someone may have experienced a similar issue with their trailer.

As for your RV "being almost new condition" a unit that sits on the dealership lot for 4 years with no maintenance may well have more problems with water intrusion, rot and damage than a unit that's well cared for, has been towed 30,000 miles or more, lived in for weeks/months at a time while camping, and carefully maintained....

I'm not suggesting that you've ignored maintenance on your RV, but at this point, we have no idea of the history of the unit or how it's been maintained or stored.