The Sterling battery to battery charger.  This is for 50Amps and suitable for older vehicles that don’t have a smart alternator.

Advantages of this unit.
1) installation: it does not get any easier. Simply connect to your starter batteries and to your domestic battery. Job done. Low costs.
2) no direct connections to the standard engine alternator , or to the outboard, if used, thus on new installations there is no extra wiring for a split charger system.
3) ensures the engine battery if maintained and well looked after
4) ensures there is no voltage rise on the engine management system , to ensure no alarms or damage to the system. Be it a outboard, or a car, or a boat
6) no vehicle warranty issues as you are not connected to the main system
7) the unit isolated both battery banks and prevents any feed back through the unit