Anderson extension

This is a simple way to do it if that works for you?
http://pages.ebay.com/link/?nav=item.view&alt=web&id=221597714097&globalID=EBAY-AU Piggy back lead

From the sound of it and it is only a guess it sounds like the car is already fitted with a VSR?

If this is correct then simply running another wiring run form the VSR to the new battery would probably achieve what is required.

If you must have the charger then keep in mind if you replace the possible existing VSR with a charger then tap into the wiring at the rear that the charger will not get an accurate picture of the state of charge of the batteries due to the length of cable concerned.

Also keep in mind that the charger will also be charging all the batteries so if you have batteries in a trailer for instance you may actually lengthen the recharge times.

Might want to look here.
https://andersonconnect.com.au/store/index.php?main_page=index&zenid=c9af71fd4b0cc8c5e3656c8eadf8eec9

The best way would be to run a minimum 8 B&S twin cable from the primary battery to the rear inside of the car, terminating in an Anderson connector.
Then plug in the Redarc to this and then to the battery box.

If you want the best for your camper too, you could incorporate a Y cable (also in 8 B&S) so you can run a patch lead cable of the same size to connect your camper to. This will give a better solution to control charging of both the auxiliary battery and the camper battery bank from the Redarc 1240 bcdc.

In any case, I would not mount the Redarc under the bonnet. Too hot in there and the Redarc dc-dc charger is not cheap.

I run a similar setup but use a Ctek D250S, which in my case is mounted in the camper boot, next to the two 100Ah batteries.
I just run a "patch lead" from the Anderson connector via a "Y" cable, one run to the auxiliary battery box, which has its own built-in isolator and another run out the tailgate to connect the camper to.