We have spent some time in Chetumal, Quintana Roo, which is on the border of Belize, but never crossed the border. So we can't speak to what it is like across the border. But the trip from the Progreso, TX to Chetumal is due-able in just about any size rig; our two rig caravan consisted of 2 37' Class A's, both pulling toads.
That said, there are a couple of things to consider. One is your timeline and the other is your route. You can do it the hard way (along the coast) or the easy way (down the interior). The difference is the coast route will beat you to death with road conditions that range from just bad to very bad to almost impossible, but some of the nice's beaches and beach towns, whereas the interior route is Tollway almost all the way and not much in the way of things to do.
We took the coast route down and it took us over 2 months to get to Quintana Roo. Of course we were sight-seeing and beaching it all the way down and we had to spend 2 weeks in Tampico waiting for parts for the diesel powered coach. But each of our stops was for at least 3-4 days to allow us time to enjoy the areas.
After spending about 6 weeks on the Caribbean beaches of Quintana Roo we headed home via the inland route. This was pretty much a speed trip after we left Mexico City due to commitments at home and it took us 5 days. Almost all of our nights were spent at Pemex stations on this route.
Next time we will take the mountain route down and the west coast back. I don't like to travel the same road twice.
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