2015 custom add liesure battery plus grey towing plug

[topshelf]

Firstly I want to add a leisure battery under my passenger seat, I get the jist of the voltage relays, but im assuming I have a smart type charging system, so I have found some relays for £95 and some as a kit for £300. if any one had fitted one can you give me the model of what you have used please.
Next my towing wiring is not standard ford kit, don't know the make but was done at a garage prior to my purchase, I need to add the grey plug for my caravan so I have reverse lights, fridge running and more importantly the caravan battery charging whilst driving. I is possible to tap of the van wiring to do this ? reverse lights I get but not sure on how to do the rest, any helpful comments I would be grateful for.
Thanks

[dumper]

I got this in January 18 Sterling 'Pro Batt Ultra' Battery-To-Battery Charger - 12V/12V 30A"
Price: £173.45 from 12volt planet it is a big unit mine is in a cupboard with good air circulation for the cooling fan it changes two large Leasure batteries I have no idea on the towing problem.

[topshelf]

Thanks dumper, I have sourced one of the units, and I think I am going to have to fit a new towing loom for the grey plug, so may as well get new type done.