Timely post, I've also planning on doing this in the spring. I have two in the battery box but want to add a couple more in the bed.
I was planning on using the anderson connector. My batteries are in the driver side wing so I may just run the cables in via the existing vent hole and connect them to the existing battery terminals. I only load/unload a few times a year so this may be the easiest solution.
I am not sure on the wirng yet, hoping to buy the exact same batteries so that i can wire them up in parallel with the existing ones without using a battery switch.
For the perfectionist the wiring won't be balanced between batteries I guess but not much choice since my battery box won't fit any more. With the heavy gauge wires I'm using (2/0) I think it will be fine.
