Jabawaki, I'm thinking you are referring to plugging in the trailer 110 V umbilical cord into a power outlet. If all is working as it should be, plugging it in will cause the power converter to energize and supply 12 volt power to your trailer, which will power the 12 volt devices as well as re-charge the battery. You do not have to flip a switch or anything to have this happen - assuming the 110 volt breaker that supplies the power converter is on. If there is a battery disconnect switch - which is used to remove all battery power from the trailer when it is not being used - the battery disconnect should be on when you are using the trailer so that the 12 volt appliances can be supplied by the power converter (or by the batteries if you are not plugged into power).
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch