Coming from old school I try not to mess with anything connected with the Truck system especially the Alternator and using the Truck batteries for anything that were not designed to be used with.
This to me is my only way to get back home so I sort of keep steered away from it. Once you turn the ignition key 'OFF' your truck battery system is designed on its own to shut down systems etc to keep the truck start battery in good working for long periods of times when the truck motor is not running.
However you can add all sorts of things to trickle charge the truck start battery on your own I reckon. The small solar trickle charge units comes to mind sitting on the truck dash. You have to plug this into a 12VDC receptacle that doesnt auto turn off however. Most of my 12VDC receptacles associated with my truck all turn OFF after a short time out once the iginition key is turned off.
Just my take on the subject...
Roy Ken