Surprised to see this attitude in this forum, where I thought by nature of the topic most people would be well traveled.I'll be interested in seeing what the experienced people have to offer.
For myself, as a true Gringo, I'd stay the heck out of Central America or Mexico in any vehicle. Not being able to speak the language, not having any idea what sort of laws or lawlessness to expect (by the police), I no longer have that gnawing of an adventure gene.
I have lived and worked all around the world but I am from South America. I can tell you from experience most people doing the sort of trip you’re planning through my country are foreign. I once helped a couple find “agua” as this was the extent of their Spanish vocabulary. Their modes of transportation range from bicycles to 7 figure MAN expedition campers. To get to where I am they have crossed all of Central America. I can’t say that they haven’t run into any problems but I am sure you’ll be able to figure it out.
To answer your question:
For my country, foreign license plates do not get subsidized fuel prices and their car themselves have a “visa” which is 3 months. So if you’re passing through, you’ll be fine. If you fall in love and decide to stay, you’ll need to drive to the border every three months and turn back around to stay legal without having to register it.
Fuel wise: diesel is usually more available, and less dangerous to transport if you decide to fill a tank to bring with you in an emergency.