Well, I got out there this morning and investigated the inside of the wall behind the crack. Nothing at all is out of place. There are no tears in the wall covering and the wall feels solid. The siding tear is the only evidence that something isn't quite right.
I'm proceeding as if it is just stress from flexing, and that everything internally is still intact (which I can't know for sure without tearing into it). I'll patch it for now using one of the methods you all have suggested. I just need to pick up a tube of ProFlex.
On a good note, while out there, I investigated the carbon monoxide detector (or is it also a propane detector?) because it hasn't worked since we bought the trailer. On the 5 camping trips we took this fall I just shut the propane off at night while we were sleeping. I didn't want to take a chance. Turns out, one of the wiring connections in a wire nut right behind the detector had lost contact. I twisted them together and reinstalled the wire nut to find that I now have a functioning detector. At least it works with the test button.