adding a 12 volt battery to a bank of 4 =6volt battery

I would be sorely tempted to wire in a smart solenoid and use the car battery for outside lighting. Like an awning rope light. This keeps the Hatfield Boy from Getting frisky with the McCoy lass. It isn't at the dinner table or out squirrel hunting that the feud would ignite; it's when they were together with "nothing going on" that starts WW III

The old Trojan car jar RV battery had a higher antimony level and would survive longest with the 5% crowd. I don't know about now.