Have you been on the roof and checked all caulking? That is the first place to look to find water leaks. The roof should be checked twice a year and all caulking dug out and replaced probably once a year.
Ken's suggestion is probably the best approach to find out what has happened inside the wall. Sometimes, a little creativity can help. It might be possible to cut a small clean hole, check for damage, and if you find none, figure how you could cover that hole repair with a picture or something else different. Gotta think outside the box sometimes in these crazee beasts.
2003 Dodge HO 6 speed Laramie Dually, Jordan controller, RDS fuel tank, Air Ride 25K hitch
Carriage 36' fiver, Onan 6500, Prosine 2K inverter, 3 slides, MORyde IS and Kingpin