To answer your question briansue, those bonfires on the side of the road are actually places where the 18 wheelers stop to have a bite to eat, they have been there for more than 50 years and seeing them from my very beginning driving to the border, you will also find them on other roads where there is commercial truck driving at night, they are also there in the day time, a make shift tent with some folks, they take your order and go to the nearest house and prepare the tacos or whatever they serve and bring them to you.
Now getting back to the fuel theft, it mainly takes place South and east of the city of Puebla big time, there are other places that it is beginning to happen, near Guanajuato, in Tamaulipas, Veracruz, Estado de Mexico, and lately near Mexicali on the pipeline that goes to Tijuana, all of this is an offshoot of the cartels.
If anyone offers you "gasolina barata" (cheap gasoline) let them know that you have a full tank and thank them. "tengo el tanque lleno, gracias"
Be aware and be safe.
navegator