One of the reasons I keep thinking she may have dumped sooner is the thought of having the evidence with her in the car and the need to get rid of it as soon as possible.
A couple of things have always niggled at me about the possibility that took all the incriminating stuff and disposed of it in the desert.
1. It's desert. in the middle of the desert anywhere near a road would have been hellish visible. Even on a remote road, in fact, the more remote, the more she would have stood out.
2. If buried the stuff, she'd have needed a shovel, and that ground is probably harder than heck. Plus, she'd have been persnickety about dirt and scorpions. Eeewww....
3. If built a fire, heck, that would be the smokiest fire imaginable. May as well send up smoke signals to a car full of passengers driving by. And, she'd have had to have the sense to remove the bullets from the gun unless she really wanted to shoot herself in the foot (wink, wink).
4. is a gal of bright white pants and layers of makeup. This is not someone who would deign to get grubby, especially if there were other alternatives.
5. The obvious option for was to leave the evidence in a variety of locations. Under conditions 1-4, this is not happening!
6. A facility with multiple dump spots, safe and inconspicuous, where a gun is normal, would be perfect. IMO. And the Last Stop has many different kinds of places to dump everything and drench the car, since it has an RV park.....
7. may, in fact, have been in somewhat of a fog during this drive (as she testified, but not for the reason she testified; but as always her lies are sutured to a reality). Adrenalin might have kept her there, as well as cruise control, focusing on her need to get to SLC ASAP, her BPD. She may, in fact, have only realized when it was almost too late, that she was about to go over Hoover Dam with all its hazards for a fleeing murderer transporting blood-stained garments and a bloody gun. The Last Stop might well have snapped her to attention, bright lights, burgers, and bullets to boot.