12V circuit only works on shore power or generator

Another possibility. Your rig likely has an auto reset circuit breaker near your coach batteries. When the circuit breaker goes toes up it disconnects your batteries from the rig. On most rigs the converter will operate without the batteries which would allow your 12 volt to work fine when hooked up to shore power. You can test whether the auto reset breaker works with a simple continuity test - they are available at any auto parts house.