I think they are most likely close to home as well, maybe within a 50-60 mile radius. This guy Simpson was a serious homebody from everything I've read. He hasn't really been anywhere or gone many places in the 30 years he's been married, according to sources close to him. It only makes sense that he would go to some secluded place that possibly only he knows about. Getting out and driving up and down the road for any distance at all is likely to wind up in a police chase, and he's bound to know that. This has been broadcast all over the radio in that general area, of which he would have also heard by now. I don't see anyway that Simpson wouldn't know he is being hunted, unless he is either dumber than a box of rocks or dead.
It's been a week now since the two of them disappeared. Carli is probably getting tired and wants to go home by now, but he knows that would be disastrous for him. With the motive behind this being completely unclear to just about everyone, it's hard to say what his decision will be whenever the food begins to run out or if they are spotted and recognized by a passerby. Hopefully the two of them will just show back up and proclaim they went on an impromptu camping trip.
I have my doubts that he ever intended on returning back home however.
What he could so, that never really occurred to me until now, is drop her off somewhere alive and physically healthy, and disappear himself. The public pressure to find him would be gone, and he could travel light alone.