SidecarFlip wrote:
I'd be investigated why it cracked in the first place. ...
OP's model is a 2009 and he's from Illinois where outside temperatures range from -20 to 105. And since 2009, we've had temperatures in the Mid-West between those ranges.
Simple age, lack of flexibility because the materials are drying out, and uneven expansion and contraction of the shower surround and the wall behind it add to the mix.
Add to that, if the surround is that plastic, fiberglass, rubbery panel material commonly used in the late 2000's, more than likely, the screw hole holding on the surround panel was so tight, it provided no wiggle room. This was a real problem with RV shower surrounds in the mid to late 2000's.
Each hole where the screws are, should have been a bit bigger than the actual screw, thus allowing for some movement due to differences in expansion and contraction due to temperature changes. With no wiggle room, the surround, being weaker than than the wall behind it, was the first to give way.
Considering this happened over the winter months, this is not uncommon at all! That's the root cause, nothing wrong with the camper. If this would have happened in the middle of the summer while driving down the road, yes.... there is probably some structural shift going on. But being mid winter in Illinois, I'd say with about 99% certainty, it's simply temperature changes and the material touching each other, drying out, loosing flexibility, and has no room with screw holes for wiggle room.
Newer camper's (after 2010) started making the shower surround holes bigger than the screws, and even later they started putting in plastic pop-rivets which provided even more flexing, while driving and due to temperature changes.
And now, within the last 3 or 4 years, they aren't even putting shower surrounds up at all, just using treated paneling that matches the same paneling as the rest of the camper. (which I really don't like).
For the OP, there is really not much you can do to "fix" the cracks unless you switch out the surround with new. My previous camper did exactly the same thing and I never attempted to install a new surround because, 1) it was just too much work and I didn't want to do it, 2) to match the dimension and the same material I had to go through my dealer to get new and the price was astronomically unbelievable! I seriously thought about make-shifting a new one with materials from Lowes, but then I fell into the "lazy mode" and decided I din't want to do this much work after all.
What I did, was like SidecarFlip recommends. Drilling a small hole at the end of each crack line so it won't go any further. I then used liquid nails behind each crack to adhere to the will, where I could get it through (and Liquid Nails still allows some softness for expansion and contraction). Once that dried, I then used a simple clear silicon bathtub sealer and ran a fine bead over each crack. It was very noticeable, but at least water from the shower didn't have any way to seep behind the plastic surround and into the wall.
After all that, I took some time to remove each screw and drilled each hole out a little bit bigger and then reinserted the screw. It never spread any further for another 4 years.
When selling the camper, I specifically pointed out to the dealer the shower wall, and he didn't seem to care. Cracked or not did not change the value of the camper in their opinion. I still got a fair trade in.
Others may chime in, but your cracking issue is not unique. It is a very common problem with campers from that vintage.
Edit, don't be surprised if the linoleum on your floors crack also. Same reason... it's fixed tight, sitting under your cabinets and such. If age causes it to dry out, and add expansion and contraction due to temperature changes, linoleum floors are notorious for cracking also.
