First good place is at our house in Santiago, NL (pueblo magico, 20kms south of Monterrey) :)

Los Lirios, Coahuila (just south of the 57 turn off from Ramos Arizpe before Saltillo)

Guadalcázar, San Luis Potosi (pueblo magico)
Park in the first town square at the church. Walk to the next plaza and let the police know you're there. They always say okay.

Parador TurĂstico San Pedro (Carr Saltillo - Matehuala)
Giant rest stop with excellent food, gas station, restrooms, and a huge parking area. Many rvers overnight there. I have pics but can't find them now.
These are just a couple of places we have stayed.