Dual Battery Newbie

[bsteveo]

Hi all.
So it's come the time that I have to replace a battery in my Patrol. I have an 06 gu 4 3.0 with a dual battery setup. I know absolutely NOTHING about the setup let alone how dual setups work and such. The only "accessories" i have are fog lights and a brake controller (previous owner had the car setup for towing by the looks of things). My auxiliary battery has been dead for quite some time now and disconnected it a while back. This has made hardly any difference to the power usage of my car. However, I've decided to move my main battery over to the auxiliary spot and buy a new one for my main power source as it feels like it may be running low on power. Then when that one goes flat I'll replace it also. I've never seen terminals in a car like mine before so I'm confused as hell as to what the 2nd battery is actually doing. According to my mechanic I don't have an iso switch so the car isn't drawing power from the aux battery when the main is low to start the car. Any insight would be much appreciated because I'm in a new world here.
I've attached some photos. The first 2 are my main battery and the 3rd is the terminals on my aux battery.
Cheers,
Ben.
batt1.png
batt2.png
batt3.png