Run Solar Circuit and Shore Power at the same time?

I learned the rule Mr Wizard mentioned when my controller made a loud and nasty alarm sound continuously until I reconnected it to the batteries. After that I hastily put a switch on the panel to controller feed.

Glad to meet you, turnthepage! I also find no need for another source of power since getting a solar panel (just 100 watts in my case). I've been feeling disbelieved when saying that. It may change if we do winter camping because the sunlight is weak and short at 55 degrees latitude.