Before you can plan a trip to Central America (Guatemala and beyond or Belize), you have to make a decison on how you are going to traverse Mexico. It is crucial that you make this choice before showing up at a Mexican border crossing. Do it wrong and you will be turned back. Period. There is a classification of travelers to Central America called Transmigrante. You'll need to decide if you are one or not.
It boils down to how much "stuff" you have with you. If you are pulling another vehicle or a utility or cargo trailer you are definately a transmigrante. If you aren't doing those things, but do have a lot of stuff becasue you will be living in C.A., you are probably a transmigrante. If you have a generator, enough tools to build a house, lots of electrical devices for the home or anything to sell, you are a transmigrante.
A transmigrante needs a special visa. You also needs a specialized broker. (Names, phone numbers and personal experiences included in the FAQ). You may only cross into Mexico at only one Texas port of entry. You have to register you and your vehicle with US authorities before exiting the USA and there is a waiting period. You have to pay a deposit. It it a PITA.