Remove the fuse from the socket showing the red light. Leave it out and continue operating the trailer. You will soon find something that is not working. At the same time put a volt meter on the battery posts while plugged into shore power. You should see charging voltage of 13.5-14.5 volts. That would rule out the battery charging section of the converter. Go ahead and operate everything you can find on the trailer, one at a time until you find what isn't working. The factory should have labeled each circuit to help diagnose the problem. Being a fuse means that it is a 12 volt circuit. 120 volt circuits have circuit breakers.