It may help if you could post a picture of it and give us the size.
Assuming that you're talking about the passthru door and not the water heater door, you can search for RV Baggage Doors and find something the right size.
If there are decals on the door it may be better to call RPod parts and see if they can supply you with one.
Salem 29RKSS Pushing a GMC Sierra 2500HD!
Gotta go campin!