Your best path to do this is to simply do a Big 3 (1/0 engine ground, 1/0 to your main battery, and 1/0 from your main battery ground to chassis)
And then to run a 1/0 from the main battery positive to your rear battery positive.
The whole point of a Big 3 is to in theory get maximum output from your alternator by increasing the wire size (causing less resistance)
I plan on doing a big 3 in the upcoming week or two.
But ok it looks like that's what I'm going to do I already have a 0 gauge run going to the rear.