Been there, done that. I have tried to find a replacement cargo door and haven't had any luck so far. Here is what I have found:
1. Depending upon the extent of damage, a RV body shop may be able to repair it and/or reskin it and then repaint it.
2. I have found a RV salvage yard in Kentucky that seems to have a fairly extensive inventory. The problem is that their customer service pretty much sucks. I've emailed them as well as called them, a few different times, and have provided the exact items that I am looking for, have attached dimensions, have attached pictures of what I am looking for, and even attached pictures and descriptions of the items off of their own website......each time they have responded and said none of the items are available or they have never responded, but yet their website seems to indicate that there are plenty of items. Maybe they never update their website but its frustrating. Here is the link to the Gulf Stream section:
https://rvexteriorbodypanels.visoner...rhomeparts.pl?
3. Another RV salvage yard but I have no experience with them:
https://colawrvsalvage.com/wrecked-rv/
If you have any luck or find a different source, please keep us posted!
Thank you,