If you want to keep it really simple and reliable and cheap then just put starter size cable from pos of battery 1 to pos of battery 2 via a battery isolater switch . You then earth battery 2 same as battery 1 ie to chassis .
This system allows youto use battery 2 as a reserve jump start battery , but it does rely on you switching to connect at start up and switch disconnect when stopped. So you need to think how reliable you are ?
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