I came across your post regarding the dark lines on your RV ceiling and I have a similar issue.
I own a 2000 Holiday Rambler Admiral 34' that has black lines across the ceiling in the front and back (soft around the area) doesn't appear to be mold or the like . I have a feeling it is a structural issue but haven't had much luck searching on the Internet until I read your post.
I would really appreciate any advice/information you may have. I hope this isn't an imposition if so I apologize.