What method/wiring are you using to ensure the inverter isn't running when you are connected to shore power?
How are you switching to inverter power when disconnected from shore power?
Do you use a generator? If so, same question applies to shore power/generator.
It is quite important to ensure you do not get a LOOP. For example: Shore power charges batteries, which power inverter, which makes 120V (which should never feed at the same time as shore or generator power), etc.
Since it is a new inverter and you've asked some electrical question, let us double check you to avoid any troubles?