Mexico
We've towed our 5th through Brownsville, down to Vericruz to Mayan ruins,and on to Cancun. We then traveled on to Belize over to Chiapas and back through Mexico city(skirt way around unless you are with the Green Angels) and central colonial Mexico. We did it once with a carivan tour and once as trainees, but didn't get back.
Buy Mexican insurance from a Mexcan company on this side of the border. Take rig titles,passports, and current auto insurance copies to obtain the permit. Go through the permit process($15). Get pesos from an ATM and travel.
Diesel is as good as ours IMHO and available only from Pemex. Roads are narrow, variable, but fair. Toll roads are better, but expensive, and you miss much of the culture. RV parks are not always available, frequently old, and in marginal repair. The people speak less english than on the west coast of Mexico, but try to be helpful.
If I were going again I would want to at least buddy up with one other couple for safety and enjoyment.