I have found very little reference to the auxiliary battery in the 2015 upfitters manuals; guess I need to do a deeper dive.

 

I use a SurePower 1314 battery separator. I think it's really good bang for the buck.

If the battery mounted to your frame is a starting battery, you are going to finish it off pretty quickly with your intended application.

The 2013 upfitter manual has a few schematics for aux battery with isolation.

 

Great information, thank you!

 

GM has done different things in different years, different wiring options. Sometimes the auxiliary battery is isolation, sometimes it is a 24/7 parallel setup. I think the key is understanding differences of each and making the right decision for your own needs, including whether or not you wire as the factory does or not. I am not sold on an schematic or parts until I actually go and purchase them. Even though I do have a relay here that would likely suffice, in the back of my mind I am subject to ordering this part for it's heavy duty aspect and it the fact that it provides flexibility in wiring up tell-tales. It provide electrical contacts to tell you when the battery is engaged and when the battery is not engaged. I am sure this can handle the amperage of a large group of batteries connected in parallel. In my grand overkill plans, I am up to four auxiliary batteries that will likely never get installed.

(Correction, I thought this was listed at 1000amps. Maybe there is a second similar model. 500 amps still more than enough.)