I don't think you realize how rural that area is. It took police four hours to get to a double murder scene! Think about that. I'd hazard a guess that you could stand on that road for an hour in the middle of the night and not see a single vehicle. And there is likely a reason the couple decided to sleep in the van rather than attempt to get help when they broke down, there is no help.
That said, I wonder if the boys came across the broken down van and thought it was an easy mark for a quick robbery, not realizing the occupants were sleeping inside. When surprised by what looks to be a very fit Lucas Fowler, they opened fire. They fled and came up with a plan to disappear across country, which they couldn't do in their broken down old truck, so they found and killed Dyck simply for his vehicle.