I've wondered this same thing. He would have taken a huge risk to take those boys with him to dispose of her. One glance in the backseat, or if she had "come to" if she was not dead, or if the kids were not fully asleep and saw something. I'm beginning to think maybe he did dispose of her quickly, then came home and cleaned up the car, cleaned carpet/turned on fans, then grabbed the kids from their sleeping beds. Car alarm could have gone off while he was hastily doing these things.
Maybe he HAD to go camping so he could burn some evidence. It would be interesting to know if the kids talked of a campfire and smores. And that quote you mentioned about mommy in the woods, strange. I recall reading that as well, but don't know of its source.
If this was the scenario, her body has to be close. I don't think he'd travel a great distance while his kids were sleeping at home.