You got the converter out before so take it out again to where you live that has 120v power and do a bench test. If it is a hard wired 120 you can use an old cut off appliance cord, wire- nutted to the 120v converter wires so you can plug it in. Use your meter to see if it is outputting 13.6 DC.
With the converter gone, and the battery hooked up do the 12v items in the rig work? If not, use your meter along the pos and neg paths from the battery to the fuse panel battery lugs to see where the 12v is not showing. Maybe across a fuse or that switch.
Unhook the battery and take it to where the fuse panel is inside and find the two battery lugs where the pos and neg wires come in from the battery tray.
Use a set of ordinary jumper cables to go from the battery posts to those battery lugs and see if the 12v things in the trailer work. If they work from there but not from the battery tray end of the wires then the short is back down those wires but you missed it.