series you'll have a 24 volt battery haha!!
parallel is what you meant, positive to positive and negative to negative
the best ground you can ever have is one coming straight from the front battery, or even better if you haven't done the big 3 would be the alternator ground
but yeah just please don't run the isolator, the way I see it with as many batteries as you are going to run you won't drain them to the point where you can't start your truck anymore unless you are sitting there with the truck off for hours playing some songs
parallel is what you meant, positive to positive and negative to negative
the best ground you can ever have is one coming straight from the front battery, or even better if you haven't done the big 3 would be the alternator ground
but yeah just please don't run the isolator, the way I see it with as many batteries as you are going to run you won't drain them to the point where you can't start your truck anymore unless you are sitting there with the truck off for hours playing some songs