What you need is a split charge circuit, a feature of all motorcaravans. Quite simply it is a relay between the positives of both batteries. The relay is usually energised by the D signal from the alternator. Your alternator should be quite capable of charging two batteries. An alternative is to use a voltage sensing relay. This simply fits between positives of both batteries but when the engine battery voltage rises due to alternator charging the relay will detect the increase in voltage and close to couple both batteries together and allow them both to charge. If you need more info or a wiring diagram just ask
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