My skylight is not only not framed in, it's also not centered in one roof panel either. It falls in between two panels, so there is a joint in the cutout.
The skylight pieces are held in place with 3/4" #12 sheet metal screws that just grip the surface skins of the panel sandwich. If doesn't seem to be a lot of holding power but that's pretty much how everything in the whole RV is held together.
The ceiling seam:
Interior view of the skylight and hole in the roof, before the interior piece was installed: