The road from Agua Prieta to the Toll Road south to Zacatecas is a two lane road with a mountainous section between Aqua Prieta and Nuevo Casas Grande. Unless you want to see the ruins (which are mostly recreations and not original) or you want to go to Mata Ortiz to see the home of Mata Ortiz Pottery (we enjoyed this sidetrip very much), I suggest the Santa Teresa Crossing near the New Mexico / Texas border just West of El Paso. Easy crossing and flat multi-lane roads all the way south. There is a Pemex on the West side of the road you can overnight camp at just South of the turnoff to Nuevo Casas Grande on the road to Chihuahua. Went inside, ask if OK to stay overnight offered to pay a little for goodwill. Parked under the lights near the store & pumps. Stay at the Casino just north of the Santa Teresa Crossing and easy drive to the Pemex. If not drive all the way to Chihuahua.
Just another choice. We were traveling North when we crossed on both routes so never had to deal with paperwork at either location.
rocmoc n AZ
rocmoc n Great SouthWest USA