Def 90 to Sankey Trailer Wiring to Charge Aux Batteries

Looking for advice.
I have a 2001 TD5 Tomb Raider 90.
I am trying to retain the TR Style etc whilst still prepping for Overlanding. I like the 90 and do not want to go up to a 110.
So I have bought a Sankey Mk3 and I am presently getting a secure top on it.
However I want to be able to charge Batteries in the Sankey whilst on the go.
Any recommendations on the wiring. I have the standard towing wiring but will I need to add to it or change it to be able to charge on the go.
Thanks guys in advance.

If you only have the black 7 pin socket (12n socket) on the back of the 90 then there is no option to charge the battery with your present wiring.
The grey 7 pin socket (12s) is the socket that allows you to charge an aux battery. (At a max of 15 amp)



I've seen it done in Australia where they have installed a split charge system and installed Anderson plugs to the rear of the car and run cables to the battery in the trailer. Like in this vid. https://m.youtube.com/watch?v=8U6Dh_214m4
Advantage of this is you can charge with a higher current, but I've never seen it done in the uk.