You don't have to have an isolation switch altho 95% or more do so that you can stop a load draining both batteries.
They can range from a simple On Off switch like a Cole Hersey, to a solenoid to a smart relay (or VSR) such as Redarc Piranha etc etc
Instrallation varies from easy peasy to medium depending on how techo the switch is and how eleccy savvy you are
Vehicle Alternators are crap at correctly charging a battery.
Vehicle alternators are really crap at correctly charging two batteries.
Most people with dual (or more) batteries will charge their batteries overnight once a month or so with a good quality smart charger (CTEK etc)
This will significantly increase the life of your batteries and for any nah sayers it sure can't hurt
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