How to Patch Wall Seems

You could add a piece of trim right over the seam. When you get done, it would look like a picture frame. You might be able to do something different inside the "frame" to make it look like an actual picture that's suppose to be there, (depending on where its located).

Most campers have a matching trim over the seams. Manufacturers don't even try to install with tight fitting joints. They just stick a matching trim directly on top of the seams and joints in the wall and corners, and ceilings, and floors.