Almost any Pemex will let you boondock overnight if there is room for you to park. Some want 5 or 10 peso's, others don't, but if they don't, offer anyway, they will keep an eye on your rig for you. Some have large truck stops behind the station with a security gate and night guard. These are always pay stops. Most also have a restaurants, a small store, and showers available. We've always found the crew and other drivers very friendly and helpful; although language can be a problem. We figured out that if we handed them a lined tablet and pencil and asked them to write it out. We could figure out what they were saying. They just speak too darn fast and don't seem to know how to slow down. The pronunciation was also a problem. Not like anything we had practiced :)
Oh, and if it's also a cross-country bus stop, it might even have a dump station. These almost always have a crew that will wash your rig while you're there. They are there to wash the buses, but also wash other rigs too.