Welcome to the club. This is a common complaint with the NNBS trucks. I have gotten the "battery low, start engine" message many times on my 2011 Tahoe when turning on the key after having the doors open just cleaning the interior etc. I also don't drive my truck every day and it will sit for a week or two and it will barely start. A couple of weeks ago I was packing for a trip with the doors open and closed sevaral times and a number of minutes each time. It had also sat for a week. When I got ready to go it wouldn't start.
Just yesterday I installed the dual battery setup just like in the police Tahoe and 2 new higher capacity AGM batteries. I now have about 2 1/2 times the battery power I did before. I didn't do the isolator setup as I didn't feel I needed it but if you do a lot of tailgating or use electric coolers etc you may want a dual setup with an isolator.
I found the exact GM battery cables that come on the police tahoe on Amazon.com for a little over $100 which is about half the price from a dealer.