Similar thing happened to my unit a few years ago. We had stopped at a Walmart and I noticed it. DW and I popped it back in and I applied a bead of clear silicone (because I had some) along where the panel fits into the rubber gasket. It has not happened again. Your panel will need to placed back into the molding around the door. You should try this yourself. I will bet that you are done with it in 30 minutes or less. I would start popping it in under the door first and work your way back.
I looked at my skirt panel where it passes a door and there is sealant along the bottom of the rubber to hold the skirt panel in place. They use Pro Flex clear in the factory (not silicone) but if you have silicone - use it.
The second pic is a sketch of the way I remember the gasket and the J-panel - just to give you an idea for how it fits together. My sketch is probably not exact.
Here is a thread I started when it happened to us...
https://www.montanaowners.com/forums...ad.php?t=74626