Well, there is a lot to your question. I agree a good book on DC systems will help you.
First, the battery charger (I believe this is what you were referring to as an "AC to DC converter") should be permanently wired to charge the batteries. This means that when the charger is plugged into AC power, it is supplying current
to all of the DC loads on your boat. This includes the lights, refrigerator
off some other postings on this, a purist may say the current comes from the batteries, and the charger recharges the batteries. Until we can trace the paths of individual electrons, this is not worth arguing about.)
The type of battery charger you have will depend on how you use your boat. I had a boat once with a Newmar charger, and it did not fry my batteries when left on. If yours did fry your batteries, you have a problem that needs repair.
Second, I understand you want cold drinks in the fridge, but a refrigerator
will quickly drain you two new batteries unless you have something to keep them charged. While at the dock
, this will be your battery charger.
So, trying to directly answer your question about the "AC to DC Converter switch" - you should not have to operate any switches on your battery charger to run your refrigerator or your lights. The battery charger you have should be able to handle variations in load (for example, as the fridge cycles on and off) without you turning it on or off. You should be able to leave the charger on without fear of damaging your batteries. If you can't do this, you probably have a problem with the charger.