Ooops - sorry - I forgot to paste the website where I found the list . . . http://en.wikipedia.org/wiki/List_of_archaeological_sites_by_country . . . and also a list a Maya sites . . . http://en.wikipedia.org/wiki/List_of_Maya_sites . . .
Vieques was great and we took Spanish lessons and also classes where we worked with Puerto Ricans with us trying to teach them English slang and then trying to teach us Spanish slang. Lots of fun but it did not take. We also took Spanish classes in Mexico. Still did not take. We have books and tapes to study. Our brains do not seem capable of new information. But we do seem to get by with our limited vocabulary.
For SCUBA diving we stayed at Paa Mul and arranged to dive with the outfit there - but we did not like their style of diving and would never dive with them again. There was an outfit at Akumal we liked and would prefer diving with them. Diving cenotes with a guide is also a great experience. I did that many years ago when there were only a few accessible cenotes but now they have opened access to many of them and have rules in place about guides and experience etc. Guides may not take more than 4 divers - obvious safety reasons.
Friends of ours stayed at Xpu Ha on the beach in a 36' motorhome but resources are limited and the water is awful. Depending on the size of your rig we think Maya Bell at Palenque is one of the most memorable places we have stayed anywhere. We boondocked in the parking lot at Uxmal with no problem.
We took the coast route down along the gulf and it was more work than fun. Many people now say to cross at Laredo and take the cuotas all the way down. That would be - 85/85D from Laredo to Monterrey - 57/57D from Monterrey to the Arco Norte ( http://www.arconorte.com.mx/mapa ) north of Mexico City - 150/150D from Puebla to 145/145D - to 180/180D to Villahermosa.
Along the way we stop at Matehuala off 57D and take a day to visit Real de Catorce - a must see. Then after San Luis Potosi we cut across 110 to Dolores Hidalgo and 51 down to San Miguel de Allende to visit that area. From there you can take 111 back to 57D near Queretaro and on down toward Mexico City. We would definitely find a way to visit Teotihuacan if you can. There is a way to get there from the Arco Norte (40D) to 132D - or 5D off 57D takes you closer to Mexico City but it is shorter - traffic not too bad if you don't make a wrong turn. A stop in Cholula to visit Puebla and the Cholula Pyramid if time allows. 150D takes you over a mountain pass that is often shrouded in clouds - but if you get a clear day there are several mountains is the 17,000 to 18,000 foot range - Orizaba - Popocatépetl - Iztaccíhuatl - among the tallest mountains on this continent. Between Puebla and Villahermosa we stayed at a Pemex as there are no RV parks close along that route.
Many people still take the gulf coast route but it can be rough and confusing - though it has probably improved some since we did it. The route I suggest is more miles and more tolls but probably faster and easier driving.
I can't tell you much about crossing to Belize or Guatemala but have heard it can get confusing as you may have to turn in your permits and vehicle paperwork at the border when leaving Mexico and then get all that stuff done again when you re-enter Mexico. I have nothing against going to other places but with the time you said you have it could be more than enough to try to see what Mexico has to offer. Take the time to experience the places and not just fly by.
I would also suggest getting the latest Garmin GPS as well as Microsoft Streets & Trip 2013 to map your route and take the time to check and double check to see that your GPS is taking you the correct route - any wrong turn can lead to problems.
