+1 for the voltage sensitive relay. I use one of these Victron units :
In my scenario I have the traditional two battery "house" and "starter" arrangement: the alternator charges the start battery as priority.
For winter storage, the priority is on the "house"battery as it has some light load (tracker).
The beauty of this VSR is that they sense charge on either side of the circuit and combine when either battery is receiving charge- so whilst the engine alternator is wired to charge the start battery, the mains charger is wired up to the house battery (via a Bulgin plug and fuse)
If the mains power drops then the VSR isolates the start battery.
No combiner switch to remember to change over.