You are somehow bypassing the inverter and connecting the breaker box directly to the batteries.
For now, I would test and simplify everything after disconnecting everything
1) Connect the inverter to the battery and then connect a load directly to the inverter, say a
light bulb or aquarium heater, do all of this in the 'garage'. With this we can verify if the inverter is working correctly giving us 120V. Let me know if this works.