JMO
The really strange thing to me about the couple in the Van is the report from a witness that they were seen speaking to or having some sort of heated discussion with another person.
That other person was either the killer(s) or someone trying to help them and not being able to convince them to leave the van there and come with them to safety or something like that.
Since they ended up murdered I would have to put the odds more on the side that the witness saw the murderer and if there were two of them then one may have been hiding or out of the persons view.
Not much to make me lean that way other than the witness seemed to think it was more than a friendly discussion.
I have to wonder if Lucas was the one outside speaking with the person and as things got heated maybe Chynna was inside with a weapon and maybe was told to use it if she needed to by Lucas. She may have seen things get physical and maybe she fired a shot out the window to try to scare off the person(s).
Or maybe she saw/heard the witness car passing, knew the person on the outside was trouble, and tried to fire that shot out the window to flag down the witness who kept driving by.
The shot through the window is a little strange because if it came from the inside then it means at least one of them had to still be inside the van.
Its all so strange and I dont like coincidences in most cases like this which I think is also why I am leaning that the witness may have seen the murderer(s).
The road worked witness saw one man talking to both Chynna and Lucas. Something about the body language caused her concern, so she did not stop.
I would be very susprised to learn that Lucas and Chynna were travelling with an illegal gun. It's one thing to take a van on a 3 week road trip with questionable vehicle registration, but an illegal gun is a whole different problem.
I'm inclined to think that after the altercation with the bearded Caucasian man, who was close to the same height as 6'3" Lucas, the second suspect emerged. All we really know about the suspects is that if they want something, they will kill to get it. They didn't want another old vehicle. They probably wanted money. Early police reports were about a possible robbery, and all ID (except Chynna's hidden passport) was missing.
The shot through the back window doesn't tell us much. We heard that there was glass on the ground outside the van door, but if the door was open when it was shot, it could have come from any direction.