I can easily imagine a scenario. They are desperately afraid to get caught. That's a basic feature of antisocial personalities (as compared, say, to people hearing command hallucinations or flying off the handle in a domestic quarrel).
So, they go back and forth, "We can't leave, what if someone saw our truck? They'll catch us right away!" Perhaps one wants to go ahead and take the truck, the other doesn't. Perhaps Kam keeps saying the truck needs to go back to his family and he can't leave it. They both know it contains evidence that will specifically implicate them in the murders. Eventually, they agree to burn the damn truck (now an immediate source of contention between them) after grabbing a new car. The plan to grab the new car reunites them and gives them energy to go on to the next phase.
Whether they discussed it or not, burning the truck and taking the RAV4 was essentially their commitment to joint suicide/potential death by cop. Death pact. That all became necessary after they killed Chynna and Lucas. It was a dramatic ploy, it was not thought out in advance.
I can imagine them weighing out pro's and con's of taking the truck. How much forensic evidence does it contain? Once they start thinking about any of the evidence, it probably becomes clear that the truck is filled with their fingerprints and DNA, no question about it.
It's also a big middle finger to their families and a billboard that says "We aren't coming back peacefully," which may have taken them a couple of days to get to. It was a really final act, they were then willing to die, whenever it happened. I'm sure they at least thought about turning themselves in vs. burning the truck...
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