I'm having the same problem with my 2012. I'm also experiencing some floor sag in those areas of stain in the galley of my 23' FC/FB, separately, I've experienced sag in the floor with no stains in the rear bathroom right in front of the dumper.. I live in eastern Washington, an arid desert, so moisture isn't normally an issue (I've also woken up to 84* mornings due to my "free electric" ceramic heaters).
I've made a claim with corporate and the dealer is in complete agreement about repair. I've wanted an Airstream since my teens and in my thirties I couldn't be happier.... Dreams really do come true...