Just for completeness, and to allow others to comment/correct as necessary, I’ll post this again here.
I can’t answer specifically about your boat but normal Bavaria practice is to have a single master switch in the common negative battery lead. Although most boat builders put the master switch in the positive lead, so long as all circuits are isolated by the master switch in the negative lead, there doesn’t seem to be any problem. The advantage is that, even with multiple batteries, you only need one switch.
Charging from the engine alternator is usually via a split charge diode unit, with the alternator sensing the battery voltage to overcome the slight volts drop in the diodes. Both batteries are charged automatically from the alternator, without human intervention (or thought).
Regarding the mains battery charger, this depends on the type of charger and how it was actually installed. Some chargers have dual outputs and these are wired to each battery (house and engine). On other installations, it is not unusual for the mains charger to only charge the house battery; the engine battery is charged, like a car, every time the engine runs. This is OK, of course, until the engine battery goes flat (for whatever reason!).
If you can’t get any info from the previous owner or the broker, then it is probably not too difficult to trace the wires coming out of the charger.
Good luck with the new boat