The starter battery negative terminal is connected to my van's chassis (as per usual) and my leisure battery negative terminal is connected to the chassis via a 20mm2 wire. I have just ordered an Orion-Tr Smart 12/12-18 Isolated DC/DC Charger; I didn't need the isolated version as my batteries are already bonded via the chassis but it was cheaper than the 30A non-isolated version. I will connect the starter and leisure batterie's positive terminals to the Orion-Tr via 20mm2 wire.
My question is, given that the two batteries negative terminals are already connected via 20mm2 wire and the vehicle chassis, can I connect the negative input and output of the Orion-Tr to the chassis/battery negative with thinner wire (say 6mm2 or even 2.5mm2)?
From my limited understanding of electrics, I would think that the Orion-Tr just needs the negative wire to be able to power itself and sense the voltage of the two batteries and the current path will be through the vehicle chassis as that will be lower resistance.
I have attached a Wiring Diagram.jpg for clarity.