I checked my inside door skin and it is metal, probably aluminium. There is a flange around the outside edge which is integral to the edge C-channel (wraps around the door perimeter). It is screwed on my door except where the hinges attach with rivets. This would be the difficult part to remove.
A quick search online and I found one instance where another RV door inside skin cracked and the cause was water infiltration and swelling of the wood inside the door. It is not clear but the poster thinks his skin is plastic. Certainly not what you might want to hear.
Here is that link.
A good check is to take off the inside window flange and you should be able to see the inside of the door at the window area - possibly you can determine the cause of the cracking, or if there was water infiltration causing swelling.
Sorry I cannot be of more help.
Brian