Several possibilities:
1. Blown fuse, in positive battery lead.
2. Bad or missing ground, between battery and trailer.
3. Main fuse blown on fuse panel
4. Bad autoreset breaker. Not all campers have one. Some campers have more than one. They look
like this.5. Bad, missing, loose wiring connections.
6. Internal short in the battery. Shorted battery will not take or hold a charge. While plugged in to shore power, your converter is trying to charge the battery, but, the battery is acting like a huge resistor, drawing every bit of power that the converter can supply, and still not charging. There's nothing left to power the lights or other 12 volt systems. Unplug shore power, disconnect the battery and leave it that way, re-connect shore power, and see if your lights work. If this is the problem, a new battery will fix it. If this is not the problem, the hunt is on for blown fuses / breakers, and bad wiring connections.
I would suspect the battery.
