if you plan on going for Ablemail AMT12-2 then it sits alongside the DC-DC charger.
it relies on the fact that the Leisure/AUX battery is being charged by either Solar or EHU and will effectively "back-charge" the Starter battery - so if you have some Solar or EHU it will work fine . .
the AMT12-2 wires alongside the DC-DC like this . . just the 3 wires. (starter batt, aux batt + Ground)
you just join in the feeds right next to the DC-DC charger.
this shows the setup.
@Ayjay This is from a feller calling himself
@Dellmassive ?
This diagram is all there is to do.
I tried copying the link but failed..
It's from [Guide] Battery Maintainer (for Starter Battery) -- How I Done It --