12 volt power failure

Well...here I am on the boat getting it ready for the Puget Sound Cal Rendezvous next weekend, on shore power, when out of the blue I realize that I have no power to the potable water pump, the cabin lights, bilge pump, the transducer, etc. etc. There is 120v in the outlets. I crawl into the lazarette and take the panel off the back of the power panel...find 110 power to my battery chargers and 13 volts coming out.....I have 13 volts to the back of the switches on the original panel that say, battery one, both, battery 2, off. The motor starts. Any fuses I could find were okay. One of the wandering dock mates said......"looks like that switch is shot". I reallize this panel has been working since 1979...but I was not working on anything electrical so I am surprised that it would give out like that. I also understand that trying to diagnose an electical problem with the limited info we have is impossible.....but maybe some of you have experienced similar failures and have an idea where I can measure my outputs to aid in the diagnosis. No lights is no fun.......to say nothing of the handiness of a bilge pump......thanks all.... Bob Connell Oly Wa.