The usual way is to put the relay coil to the white wire (it's white all my V40s) on the alternator. This is the wire which does the IGN lamp on your dashboard. It extinguishes the lamp when it goes high (12v), which is enough to throw your relay, connecting both batteries for charging.
This method will keep the caravan battery disconnected until the engine is ACTUALLY RUNNING (not just the key turned on) and also ensures the batteries are isolated from each other for starting the engine.
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