OK, lets start at the beginning.
When you say the lights work are they 110 volt AC lights and no 12 volt lights work?
If yes, then:
Is your 12 volt battery installed, known good, and is it charged?
A shorted out, completely dead (less than 6 volts on a 12 volt battery), or frozen battery will act like a direct short to your converter and prevent shore power from being converted to 12 volt DC. REMOVE the battery completely and unplug the camper. Wait a few minutes and plug the camper back in. All 12 volt devices should work. Replace battery.
Let me know if this fixes your problem.
If No, and they are 12 volts we need to re-attack as this would be a head scratcher.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW