A musty smell is indicative of an ongoing leak that may have lead to the growth of mold and mildew. Leaks are tricky and no one on here will be able to armchair diagnose it. But leaks can literally come from anywhere in an RV. The roof , tops of slides, windows down into walls, I have even seen water trail down the outside wall of a slide and run into the unsealed under side trim of the slide and into the floor of the RV. And often those only happen when conditions are just right and the Rv is at a certain angle. Not sure how often you maintenance your RV but the first thing I would do on a used RV is re caulk the roof and seals. You have an 11 year old Rv and who knows what sort of maintenance it had before you got it. If you can't diagnose it yourself, it may be worth taking it to a professional to have it leak tested. Beyond the damage you can't see that will continue to get worse, you may have a mold problem which is a health hazard. Good luck.
Also, if I remember, you just put a dryer vent in right? Maybe start there. You also can't rule out plumbing.
