So my wife had to hop on a PTA call, so I ended up with more tinker time. As I suspected after I posted, the heavier gauge ground was just the jumper from the other battery, and the other "new" ground was from the amps. I was able to test battery positions once the grounds were all in circuit and before I connected the second battery positive terminal. I had correctly guessed battery 1 and 2 positions and they are both in their new homes, cranking on 1 and deep cycle on 2. That being said, I'll leave this up for any other helpful tips