I am a quasi-expert on this!!!!! Yes they do crack, but no one has been able to explain why. I have theories, but I'll keep those to my self. If your trailer is beyond the warranty phase, get ready for serious expense shock. In order to replace the plastic panel, they have to pull the slide out from out of the trailer. Cost runs around $1200, give or take. Keystone considers this 'cosmetic' so it's only covered for 90 days, though I'm sure they would replace it in the first year if you made enough noise. The kicker is there is no guaranty it won't break again.
The panel serves no purpose other than looks. When the weather gets warm again, I plan to fill the rough plywood (you'll see it under the plastic when you remove it) under the panel with a wood filler, sand it smooth and paint it.