the way he's talking about hooking it up will work fine with no isolator needed. an isolator is for instances like winching, the winch is hooked to it's own battery so the battery needed for the engine isn't run down winching the truck. the vehicle can then start and charge both batteries through the isolator.
either system works fine, it just depends on your needs. if you just want extra capacity for lights, and they won't be on when the truck isn't running, the way you described is fine. if you like to sit by the campfire with your radio on until you pass out, hook the radio up to the second battery and get an isolator.
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