Doesn't have the factory door? Interesting...can you post a picture or two?
Don't know about your exact problem as I've never torn an RV door out, but if you are close to a RV graveyard maybe go take a look at a damaged one. I would guess that the header supports or indicators are still in the frame and you'd need to put a new header over it. Or it's still up there and they just added a 2nd header for the smaller door when it was installed.
What the heck, just tear into it and take pictures so we can follow along. Easy for me to say since it's not mine.