We rook our 22 foot class C everywhere, paced, unpaved (not if we could avoid it,) and we saw the country for 23 years. Now we have a 27 foot Safari Trek, Class A, and love it, and wouldn't hesitate to take it anywhere.
However, we finally found paradise, and after spending 6 months a year in the RV for 6 years, we bought a house and are staying forever.
Any size RV, up to at least 35 feet would take you anywhere in Mexico in our opinion.
When in doubt, look at the Mexican buses. They go everywhere, anywhere, and normally, considering the miles they rack up, are in extremely good condition!
Drive what feels good to you and go. Explore this excellent country!