@travelvolts can supply you a suitable kit for this.
The VW option is no good as it is still relay based. For a twin battery set up I recommend the Redarc BCDC1240-LV Electrics | Travelvolts
The 1240 would be better for a twin battery set up but for a single just use the 1225. No problem with factory swivels.
I don't think you would benefit from having a smart pass on a T6, in fact, it could end up damaging the leisure battery by letting the high voltage generated under braking straight through to the battery. I would go for the Redarc 1225 and fit the mains hook up kit with a Victron IP65 12/7 Blue Smart battery charger.
Electrics | Travelvolts
Travelvolts fitted a elogic charger in my T5 a few years back and it works fantastically. I'm going to be away in the van again this summer but will be stationary and off grid for a few days. Am I correct in thinking that on tick over alone over the leisure battery will not charge as the alternator will only be charging the main battery? I'm only running a compressor fridge off the leisure battery which normally lasts about 3 days so wondering if I can top it up by running the engine? I'm guessing not?
Have you read the first page of this? How To Fit Travelvolts Dc-dc Charger Kit as I explained the downside of the relay VW are still using, I agree that having the second VW battery option is a great start as it gets all the wiring connected up for you, but you will need to replace the relay if you want optimum charge and more capacity (runtime) of your leisure battery.I have a plug in voltmeter...lower cigarette lighter for main battery....12.5v......top dash 12v outlet....12.8v