Andrew, I think you have diagnosed your issue correctly. The pass-thru relay inside of the inverter allows the shore power to go to all the inverter duplexes. With no shore or generator power, the inverter creates the power and sends it to the same duplexes. If you remove the small cover on the inverter where incoming romex connects to the terminal strip, check with a meter 120 volts coming in and then the terminals that are the output. If no output, relay is adios. Fred