Another happy Traxide customer here. :D
Tim and Scott have answered it all, but one additional point ............
In your original post, you asked "Is it ok to run a single 6 B&S cable to the rear Anderson plug and connect an earth at the back of the vehicle...if so is there a preferred earthing location near the towbar area? Or is it best to run twin core directly from the pos and neg battery terminals to the Anderson plug."
NEVER, EVER connect ANYTHING (especially jumper leads) directly to the negative post of the original (cranking) battery in your D4. This is a sure fire way to cause some potentially serious problems. Always use a body mounted earthing point (there are two on the side of the engine bay just in front of the battery / fuse box, and if you need a larger one, Tim can sell you a stainless steel adaptor with a large earth post which bolts directly to these two original posts).
As far as running a third battery in the back goes, I also run an ENGEL portable battery pack in the rear of mine as suggested in Scott's post. I simply plug it in to one of the two permanently powered 12V sockets that I installed as part of the Traxide kit installation, and this has always kept the third battery charged without problems. I run an ENGEL 40 litre fridge permanently in the back of the vehicle, connected directly to this third battery. When I plug in my CTek battery charger to the Anderson plug at the rear of the vehicle to top everything up once a month or so as recommended, this charges all three batteries. Similarly when camping, I plug an 80W solar panel into the Anderson plug and it charges all three batteries as required.

