QTLA9111,
I much agree with your points. Some areas are safe and the libres are great in other areas the cuotas suck. I look for a comedor that has a packed parking lot during the hours of comida.
Usually trucker's restaurants are good. Depends. If I travel in a mountainous area and the only restaurant that has truck parking has trucks that's no guarantee that the food will be anything more than mediocre. But if just one restaurant among many has a lot of trucks, the weight shifts in favor of getting a heck of a big meal at a modest price. Many of these kinds of restaurants have a buffet style comida.
Bet you wouldn't pass up a lady selling a bucketful of steaming tamales in a gasolinera would you? :)