This is the first time I've read about this, but my initial thought was that he was killed first, and she had taken the truck to try to get away, went down into the embankment and tried hiding in the bed of the truck. That idea has a lot of holes in it though. They could have thrown his body into the river, but I think it would have been found by now. If they took it with them to dump somewhere else, why not take hers too?
I haven't read all of the links, have they said where she was shot? (where she was found, in the cab of the truck, outside the truck or somewhere else.)