A few years ago Tiffin's had this problem. In some cases the moisture was blowing the in-line fuse feeding these lights. Finding that fuse was a PITA on some rigs. I had 3 bad ones on my Bus that I replaced, but never replaced the fuse as it didn't blow.
2013 Tiffin Allegro Bus 43QGP
2011 Chevy Silverado 1500 Toad