For my motorhome the onboard generator has precedence. If the generator is running and I plug into shore power nothing happens. The transfer switch stays connected to the generator. If I am plugged into shore power and then start the generator the transfer switch will connect to the generator once it comes up to speed. As mentioned, when connecting any power it's best to have everything turned off. If for some reason there is a failure that allows a connection between generator and shore power the worst that will happen is a breaker will trip.
