My experience is in Southern Arizona, and limited at that. To me, the key is that you said you are staying on paved roads. If you go off road, that can change things a lot. Like completely.

So, for 100% on road, I think your in the ball park. Maybe a bit short on water, assuming a more than 1 day stranding (which I doubt on a paved road). You might also want something for shade. If the car breaks down, you will not want to sit in the car or the sun until help arrives. The tarp will probably be usable for this, if you can rig it.

Stay with the car. Don't go off trying to walk out.

Enjoy the trip. The Southwest is one of my favorites!