I've traveled extensively in Mexico and Guatemala. Fuel is not a problem, only pricey, cheaper in Mexico though. On the major highways in Mexico truck stops are often found for overnight stays. If you're heading down the east coast of Mexico be aware that speedbumps, "topes", are everywhere, and are particularly concentrated in and around Veracruz. You'll find these things in the middle of nowhere with the only building around being someone's house! And they usually come in pairs and fours. Driving is particularly hazardous at night since cattle and other large animals are likely to be found on the roads (usually not on the major highways though, but you can never be sure).
In southern Mexicio, particularly in the state of Chiapas, be prepared for frequent stops by Immigration (Immigracion), Customs (Aduana), the police (policia), and the army. Stops are more frequent heading north. I was stopped 12 times within three hours of leaving Tapachula bound for Veracruz. They're looking for guns, drugs, and rebels, especially in Chiapas which is a hotbed of revolutionary activity. Yes, the soldiers will be armed with automatic weapons, but you have nothing to fear unless you're a rebel or running contraband. And it will help if you know a little Spanish, or at least try to speak it, the locals are more than willing to help you with your Spanish.
The roads in Guatemala are a bit better than those in Mexico for the most part, and cleaner as well. Even some of the major 4-lanes in Mexcio can be quite rough. The road ARE well marked though, at least the major roads. For Mexican Insurance I used Sanborn (google them), they also have some great guide books for Mexico with mile markers and places of interest, I love 'em!
I'd suggest getting your pesos before entering Mexico, and as you head south you'll need each country's currency once you cross their borders. Banks can supply the local currency and most have ATMs that will work with your Visa or Mastercard. At the border between Mexico and Guatemala you'll find lots of money changers with huge wads of pesos, Quetzales (Guatemalan currency), and US Dollars. Their rates are not as good as the banks, but if you need a few pesos these men are the ones to see. But be careful, some of the more unscrupulous ones will quote you one rate and give you another thinking that the stupid gringo won't catch it. Make sure you agree on a price BEFORE you hand them any money, and make sure they count the money into your hands slowly. If the Casa de Cambios in Brownsville, Texas are giving you 11 pesos for the dollar, expect the money changes at the Guatemalan border to sell their pesos for 10-10.5, and if you're changing your pesos for Quetzales, get just what you need at the border to avoid taking a loss on the black market exchange...you'll need 10Q to clear in, and then when you get on down the road find a bank in Guatemala City or HueHueTenango for a better rate. One last thing, don't hang around the border town (at the Talisman Bridge) just outside Tapachula at night; it's not a good spot for gringos at night, or some locals for that matter.
I hope I've not said anything to deter anyone from traveling here. The countryside is beautiful, the people friendly, and the shopping outstanding.
Steve