I agree. The witness saw the man talking to the two victim, so they were outside the van and presumably wearing something on their feet because of the gravel. I'm wondering if the altercation with the suspect happened first, then the suspect came back later with a gun.
When found, the back window was smashed, Lucas was without shoes and Chynna was wearing one shoe. Smashing the back window suggests that the suspect broke into the van, which would happen if they were locked in the van to sleep. The side doors were found open as well. Lucas was a short distance from the van and Chynna was 5 meters from the him.
The first witness at the scene said that their body positions were identical, which he found odd.