A lot of questions. I've done the trip you propose both ways, going South and North. Brownsville is a good crossing point, in my opinion one of the least chaotic. Once across, follow the Caribbean coastline all the way. Avoid Mexico City, too many hassles. Be sure to stop at Palenque, Chichen Itza, and Tulum for great ruins. Stop whenever you're tired at relatively inexpensive hotels or motels. Very few campgrounds. Learn some Spanish. Much appreciated. Smile a lot. Bring a bit of money. Watch your belongings in Belize City. You may want to go out to the keys for some diving. Be careful with where you're going to leave the bike while away. Otherwise, you're in for a treat. A great ride. Pick up a free map of Mexico from the AAA. Great for figuring out distances. Forget GPS; another gadget to watch not to get ripped off.