Travelling across Mexico
I have spent many years travelling in Mexico, from end to end, and side to side, by aventon (hitch hiking), to 2nd class bus (with the pigs and chickens), and car. The best shade-tree mechanics in the world live in Mexico, and she is in for the adventure of a life time! FWIW, the roads in Chiapas and Quintana Roo are quite good, but have thousands of little topes (speed bumps). On one road in Chiapias I counted 120 in less than 100 kilometers... :-) On my last road trip to the Mayan ruins of Chiapas my only problem was parking too close to the wall of my step-sister's driveway, which pissed off her brahma bull who subsequently kicked off my side-view mirror!
Taking 25 gallons of US water for her mother! There is plenty of good bottled water available there. Some water for the van might not be a bad idea, however!