All the bavarias I have been on have a single battery on/off switch and like yours mine is under the chart table, however the engine battery is only used for engine starting. This is achieved by a factory fitted split diode which is in the engine bay. Mine is on the panel behind the engine and to port of the engine. You can therfore relax about using the fridge.
We have a smart charger for use from shore power and an Adverc control unit fitted to the alternator circuit which allows the batteries to make better use of the charging available from the alternator. We have an engine start battery plus 2x140amp hour domestics and we monitor battery use so we can see how many amps/hour every piece of kit on the boat is using.
Our fridge uses 2.4 amps/hour when pulling down to temperature and 1 amp/hour therafter. By comparison if you switch the cabin lights on they use 0.9 amps/hour each and you could have 12 on. This is the best argument i can give for replacing the halogen lights with LED’s.
If you do fit a device that allows the alternator to charge beyond the normal voltage controls then you need to fit an additional switch to take the engine battery out of the charging circuit when motoring for extended periods or it will get overcharged and shorten its life.
The original owners manual has all the electrical circuits included and I think it is available on this website.