Evy. If maximizing alternator charging is a desirable goal for you.....
On your diagram above you have 'Existing wire' going from 130 amp alternator to original battery.
This existing wire already has a fuse in it somewhere.
Then your diagram takes power from original battery (+) to feed 200 amp isolator/relaty/solenoid/isolation device and has another fuse, as it shoud.
But, this Existing wire was never designed to pass the additional current required when a depleted set of GC batteries is added onto the end of the circuit. it becomes an electrical bottle neck, and might blow the 'existing wire's' fuse. This alternator load dump might fry the diodes in the alternator.
IF you INSTEAD, take power for isolator not from engine battery, but from alternator(+) stud, you eliminate the too thin 'existing' wire from limiting house battery amperage, AND you also likely have a shorter circuit, AND one which does not require a Fuse as you ony need to fuse close to the battery, either battery.
So take power for isolator from engine battery = fuse required, longer distance, more copper length, and existing wire acts as bottle neck, limiting charging amps to depleted house batteries.
Take power instead from alternator (+) stud, and one bypasses the too thin existing wire. A shorter circuit with no fuse required at the alternator then goes to isolator, and then to a fuse, then to the house battery(+)
Much shorter circuit, much more effective when batteries are depleted. This orientation might also allow the vehicle's voltage regulator to hold a higher voltage for longer, since the fully charged engine battery is not 'in between' the alternator and the house battery.
Also if the fuse on the house battery circuit blows with the engine running, there is not as much of a load dump situation for the alternator's diodes to tackle, as there is the 'existing wire' to the engine battery that is still carrying the current to hold that battery at 13 to 14.x volts, and run the engine/ chassis electrics.
So UNless you are intentionally limiting alternator amps into house battery, or the back of the alternator is so incredibly difficult to reach to stack another ring terminal on the (+) output stud, it is much more effective to take power from alternator rather than engine battery.
Also if you ground house battery to nearby frame, then you should also add a ground from frame neary alternator to alternator mounting bolt or (-) stud, as the current for house battery will have to go through the original grounds, which would otherwise be another bottleneck when the house batteries are depleted.
When the alternator has a thick copper path to feed depleted house batteries, it can get very hot. Especically when Idling and not moving. Very platform specific, but once engine is upto full temperature idling, I would not Idle any longer with a depleted battery bank, but start moving, as the alternator might very well be flirting with 220F after 10 minutes at max output and that is their temperature tipping point that they prefer to remain under.
So if one intends to idle for long periods to recharge, then intentionally limiting alternator amperage into house bank by thinner longer copper is acceptable for the life of the alternator, but at the expense of the batteries which will recharge significantly slower due to increased resistance in the circuit.
Taking power for isolation device from engine battery is not wrong. Just bypassing the Existing wiring is Usually a much shorter circuit, that can yield significantly more alternator amperage into house bank, and eliminates one of the fuses otherwise reqired, and might be able to prevent an alternator load dump situation from frying the diodes in the alternator
