First post and pretty new to all things Bongo, so please be gentle

Have a Diesel yr 2000 Bongo which has 2 batterries fitted.
It would appear that they are the Japanese cold weather spec fitment
The main battery is a 72 amp and the backup an 85amp which now after removing have found to be a lesiure battery.
I have bought a new made up loom to connect the lesiure battery to the fuse box and a seperate kit to charge and isolate the leisure battery so radio and sockets run direct from the leisure battery.
However cannot figure out how best to disconnect old wiring to leisure battery from main battery to stop main battery being discharged, but ensuring everything else will still work!
Finally will main battery need to be upgraded to something a little more meaty say 85-95 aph