Here is my google research result about dual battery setup:
1) Primary and aux battery connected in parallel WITHOUT an isolate relay
When the engine is NOT running both batteries supply all OEM, aftermarket (upfitted) and trailer electric devices same time; when the engine is running, both batteries get recharging simultaneously. Also, both batteries supply electric power to starter simultaneously when cranking engine.
When the batteries run down, no more cranking engine.
2007 - 2014 PPV has this setup.
2) Primary and aux battery connected in parallel WITH an isolate relay
a) When the engine is NOT running primary battery supplies all OEM electric devices, aux battery only supplies aftermarket's (upfitted) and trailer's electric devices; when the engine is running, both batteries get recharging simultaneously. Also, only primary battery supplies electric power to starter when cranking engine.
The aux battery only duty is to supply electric power to aftermarket's (upfitted) and trailer's electric devices when the engine is NOT running.
When primary battery runs down, no more cranking engine even the aux one still has sufficient juice. The working around is exchanging primary battery's position with aux one's.
2015 - up PPV has this setup. But some police departments by-pass the isolate relay intentionally to bo back to 2007 - 2014 setup.
b) When the engine is NOT running the primary battery supplies all OEM, aftermarket (upfitted) and trailer electric devices; when the engine is running, both batteries get recharging simultaneously.
The aux battery is dedicated to supply electric power to starter when cranking engine.
When the primary battery runs down, the aux one still can crank the engine.
All above 3 configurations can be done by one isolate relay setup. The isolate relay has OEM and aftermarket styles.
If your PPV does not have an isolate relay, you could install an OEM one, but you need digging into PCM box to get ignition signal. See the following link:
www.silveradosierra.com
Or go to simple venue, install an aftermarket isolate relay. See the following link:
This is not my pic, but the parts circled were replaced by the Keyline VSR.
www.tahoeyukonforum.com
So, make one choice to meet your best interesting.