I don't think Barrentine saw the murder or participated in it (I don't think he drove the car to drop off... I think if anyone we've heard about did that, it's the guy who moved to Michigan days later).
Here is what I think Barrentine actually did. He saw the girls, maybe more than one place, but at least at the Big/Little. I know the woman who gave directions never mentioned seeing a guy around, but maybe he was just somewhere he could see that parking lot rather than among the four of them. He may have looked over there out of curiosity as the store is closed and there are somehow four people and three vehicles in the parking lot.
But, this would have allowed him to see the girls, their outfits, and their car. And to believe there might be something of interest on the security tape.
He might have seen more or known more than that, but whatever it was, it wasn't enough to worry the murderer. I think this because I don't think anyone would trust him to keep totally silent if he really knew anything.
Maybe he saw the girls or the car again that night, but again, not in a situation that would worry the murderer. There is a story he behaved strangely when he got home, but that's a little vague. Apparently, he told the police a few different stories that didn't check out. So, forgetting about him for now...
The car and its lack of gas is bothering me. But, maybe J.B. and Tracie did manage to get to some sort of event that night. There were at least 5,000 field parties taking place, after all. Obviously, they can't have seen a tremendous number of people, but still.
Meanwhile, the car gets driven by someone who is not J.B. (probably without permission) while J.B. and Tracie are at whatever event. But, this person is caught using the car and J.B. sees WHY they were using her car and gets mad. As in this is drug related (I am not saying J.B. or Tracie used drugs or knew about anything like that happening).
P.S., I still don't understand why a truck is parked at the gas pump of a closed gas station. And whatever it's doing there, imeans it could block the view we get on the tape.