He is absolutely correct. The crack was caused by extreme cold. My Springdale did the exact same thing. It is NOT because of water.
In the extreme cold winter, the vinyl surround contracts, but the screw holes do not allow it move. Something has to eventually give, and the weakest point will break. The vinyl is not near as strong as the screw or the wall, so it literally shrinks until it can't shrink any more. Somewhere, something has to break.
The mistake the manufacture made was to not drill a hole in the vinyl surround a bit larger than the screw, giving the surround a bit of "wiggle room".
I never did fix mine, and it happened in the 3rd or 4th year of ownership of that camper. I simply ran a bead of clear silicon caulk over the rip and a bead inside the rip along the wall. Worked ok for another 4 years until we sold the trailer. I never had to re-caulk the seam, and it never ripped any more.
So yes, your "guy" is correct. It IS because of extreme cold.
My current Outback has plastic screw-snaps with "give" in them. The surround is not solidly affixed to the wall. It has give in it. So manufacturers started installing later models a little smarter.
Edit: and if you notice in the photo, the crack starts at the screw hole along the top toward the next screw hole behind the shower head holder where it has less of a chance for contraction and expansion. Mine ripped very similar to yours there, along the top.
