@Smirke The OEM batteries are strong enough in general, but the life of them might end up being a bit less than if they were coupled. But like any battery, it comes down to how many amps are being drawn at any one time, and for how long.... I've factored that into my calculations and am satisfied to simply replace them when the time comes with a higher CCA on the starter, and high AmpHour on the auxillary.
@Holden_man @dugite57 I have just a standard isolator, albeit a 500A marine grade unit, rather than dc-dc. For just separation, an isolator is fine. For better 'battery management', solar connections etc, definitely a dc-dc unit.
With the right isolator, it will join the batteries for starting so in cold environments you can still have the benefit of two batteries, and then split apart again once suitable for auxillary uses. The alternator charges the main first, then the isolator switches to charge the auxillary once the main is good - the point being the main is always at an optimal charge and not being drained by accessories like it would if not separated.....
FWIW I had a traxide kit in the D4. Tim Sanderson who makes the kits was absolutely brilliant to deal with. As mentioned elsewhere I had a number of issues with the batteries and power drains etc, and he always bent over backwards to offer advice and troubleshoot, including those occasions when on the road and called his mobile needing urgent assistance to resolve something. His knowledge and experience outweighed others by far. However...although I ummed and ahhhed a bit, I wasn't convinced to put his kit in the 200 this time round. There was something about it I just can't put my finger on that caused me to hesitate.....as stupid as that sounds..