RS-485 is just not that complicated. It's a brain dead TWO (2) wire serial connection that can be run in parallel. You don't need the ground. . .
View attachment 193384
$12, Electronics toolbox item. Sacrifice an old Cat-5 cable Should be 1-8: ORT-ORG-GRT-BLU-BLT-GRN-BRT-BRN
Download the tool from eg4electronics.com, plug into your laptop, ORT-(B-) ORG-(A+) (verify pinout is 1-B-, 2-A+, if wrong swap the wires, it can only be one way or the other) verify the batteries are talking, get the readings from the tool, then use the tool to upgrade all the batteries.
Once you have confirmed the pins get an RJ-45 screw lug
View attachment 193396
snap it into the inverter in the battery hole. Verify the pins, I think it's 7-8, So ORG-7,ORT-8. You just spent $5000+ on a bunch of hardware, spend an hour and figure it out. Once you have the pins nailed down crimp a cable, or build a pigtail into a biscuit jack, and run a straight thru cable back to the batteries.