Had something similar on our '09 Flagstaff 425D recently - the exterior skin on the rear curb side seemed to bulge outwards/pop out. Looking at it from underneath, there seemed to be a line of staples (under the trim at the bottom of the panel) that help hold it fastened into the wood floor underlay just above the frame - but the skin had pulled away from that and pulled the staples out of the wood.
Since the staples were still secured thru the fiberglass, I was able to gently bang it back into place with a mallet (not a hammer) - shifting the staples a bit so they're not back in the same holes they came out of.
Not sure why it happened, but I figure if it happens again I'll use some strong adhesive in conjunction with the staples (something like Lepage PL Premium, since it seems to be compatible with fiberglass).
Mike & Kari
2009 Flagstaff 425D
2005 Avalanche ZR71