FWIW I use a Blue Sea Systems marine battery switch combined with one of their Automatic Charge Relays (ACR). Can buy them together in an "Add-A-Battery" kit, the switch supplied in the kit with the 120A ACR is the big unit and I prefer to use the smaller switch on ground vehicles. If the big switch works then:
https://www.westmarine.com/blue-sea-systems-mini-“add-a-battery-system-14152565.html is one source. The small kit's ACR is only good for 65A.
https://www.westmarine.com/blue-sea-systems-mini-“add-a-battery-system-14152565.html Can no doubt do better than WM's prices.
I place the two of them close to each other so that I can just connect the two battery wires from the ACR to the switch with a minimum of wire length. I leave the selector set on #1 and I put all of the OEM draws on the supply stud and all of my house demands (stereo, race radio, Nav, Fridge, power points, etc.) are wired directly to the #2 battery. That makes a self jump-start is just a twist of the switch away (always cautious to NEVER cycle that switch through the 'OFF' position when running or even IGN turned on), and when winching I can combine both batteries.
I had issues in the past with an OEM GM dual battery wiring scheme so now I bring the ground cables from both batteries to the
same bolt or stud - where ever the OEM ground point was for the original battery. That GM system had both grounded to the engine block, but on opposite sides of the block. That proved to not be good enough, too much resistance, and it set up a mutally destructive self-discharge cycle.
I've been using Carolprene welding cable for the high amperage cables for about 15 years. I've had none of the issues that the Chicken Littles' of one forum predicted that I'd have. Hex die crimped lugs with adhesive lined heat-shrink bridging from the insulation to the lug body. I'm on my third project vehicle wired this way (still own two of them).
ACR & Marine switch on the Blanc-Oh!:
Articulated second battery mount as I wasn't sure how much the C-channel frame was going to flex. Turns out that it moves a LOT! the Harley strap is a secondary hold-down that also holds the positive post's emergency insulator (part of a Delo 400 jug - hey it's free LDPE) in place.
