Welcome to WRV's good workmanship. Lots of folks have the crack at the bottom/rear of the bedroom and vanity slide. WRV did not reinforce the wall enough is my guess. I had both slides showing cracks, had the wreck, and the body shop fixed them, I think one of them is showing up again. If it really bugs you, drill a small hole at the furthest point of the crack, and that should stop it from walking more. Fill it up with gel coat, and repaint if necessary. I put clear silicone caulk over mine, and let it be.
Monty & Janet - 2007 Alpine APEX 40 MDTS
S/N - 75715 - Retired - Master Certified RV Tech