The diesel trucks have the two batteries connected at all times because of the high compression starting requirements.
The gasser 2nd battery is disconnected with a relay when it is turned off thus running things when the engine is off can't drain the primary battery.
I installed a DCtoDC Victron Orion 50amp charger same as someone else did above using the battery kit that can be ordered from GM. It contains the needed nuts, fuses, etc for auxiliary use cases. Worked out really well for me and looks factory.