Bummer about the wiring problem. You most likely identified the problem ... a short somewhere in the ceiling light circuit. My old rig was a 2000 SeaView with that same 12V distribution board, so I've been in it a couple times.
Troubleshooting. Pull the F9 fuse first. Then start mapping what loads are on that circuit. With fuse pulled, start turning on/off all lights and accessories. If they turn on, they are not on that circuit. I think that circuit has ceiling lights in the front cabin area, the galley area, hall and bedroom. I think the wall sconces by the booth and couch are on another circuit, as well as the bedroom reading lights. I think the bath lights are on another circuit also. But you should verify that.
Once you know what loads are on that circuit, you can guess at how the wiring run was installed. It will likely be a pair of wires (blue and white) fed from the 12V panel, up the wall into the ceiling, probably starting at the light nearest the windshield and then running straight down the ceiling from front to back, catching all the ceiling lights in parallel, one after another.
I would pull down that light fixture nearest the windshield and expose the wiring behind it. You should have a wire nut connecting the hot (12V) wire from the panel, a hot (12V) going to the next ceiling light, and a hot going to the light fixture. Similarly, there will be a second wire nut connecting a ground wire in, a ground wire going to the next fixture, and a ground wire going to the light fixture.
Disconnect the both wire nuts and remove the light fixture. Separate all the wires and plug in fuse F9. If it blows, your short is between the 12V panel and the first light. If it doesn't blow, the short is "downstream" of your hot feed. Temporarily wire the ceiling fixture to only the incoming hot and ground to see if the fixture has a short. If it works OK, pull F9 again, reattach all the wires like you found them.
Move on to the next fixtures downstream, one at a time, and repeat the above procedure until you find the problem area. Chances are, something is shorted at a connector, rather than in the wall.
Good luck on your hunt!
Karlos and Dawn
2007 Dolphin 5342 Workhorse
2013 Fiat 500C Toad