The Internet, it's information resources and instant communications has literally revolutionized travel in Mexico. Wherever there is a telephone line, an Internet connection is possible (but sometimes not economically feasible).
Presently it requires extremely expensive equipment to erect and connect a service provider to telephone lines. Telefonos de Mexico, the giant, private conglomerate, is heavily taxed by the Mexican government, and while local calls are fairly cheap by US standards "local long distance" charges can be as much as the equivalent of fifty US cents per minute.
For these reasons, an overwhelming majority of service providers are located in the more prosperous cities. As one ventures South of Mexico City, service providers become scarce except in the larger cities and tourist areas. So, it is the presence (or absence) of a local ISP that dictates the availability of an internet connection to a telephone line in Mexico.
Most RVers tend to choose a destination in Mexico in an already established tourist area. Modem friendly connections are still very scarce in RV parks. Getting a telephone line installed in a reasonable amount of time is next to impossible anywhere in Mexico -- three to four months would be an average wait unless the machinery were to be greased with a handful of banknotes.
This leaves the Internet Cafe, as the alternative. Mexico has literally exploded with new internet cafes in the last two years. Even tiny Mulege, where according to friends I dropped out of sight for a half year in 1999, now has an internet cafe.
Bringing your own laptop computer to an internet cafe is possible with larger cafes that are staffed by a "technician" who can help you connect and get online with your computer. I happen to use Earthlink as my service provider, and Earthlink has a recipe that allows me to access my email account from anywhere in the World. You should familiarize yourself with how to do that with your email before you depart.
My opinion is that it may be easier for me to not try to connect my own computer in Mexico but rather sit at a rental machine in an Internet Cafe and do my mail and Internet Browsing. Really upscale Cafes in popular tourist areas may charge as much as six dollars US per hour, but three or four blocks away I can usually find another Cafe used by Mexican students at half or third the cost.
Most Mexican Internet Cafes use dial-up connections and connection speeds can be agonizingly slow. The Mexican keyboard differs slightly from ours (US and Canada). For instance in order to access the @ symbol used in addresses it is necessary to simultaneously hit the ALT key and an unmarked key (ask for the recipe for this action at the front counter).
I feel more comfortable wiping my cyber footprints clean after finishing with an Internet Computer for the day. By clearing the browser history, I will thwart a later attempt by someone to see where I've been. If I were to for some reason access a sensitive bank or credit account online, I certainly will educate myself as to taking necessary security precautions before I leave for the trip. Rather than access very sensitive areas I would rather my bank send me an email regarding problems in my credit card account if one should arise.
As an aside, Cellular telephone communication is big business in Mexican cities and resort areas. Your telephone may have to undergo a chip replacement in order to function with a particular cellular provider, but the good part is that most of the services are prepaid, and are extremely easy to get connected to. Local calls are very affordable (roughly eleven US cents per minute), and can be very useful when communicating with a spouse or friends.