You might want to check out the travel club
Vagabundos del Mar website for tours. Their tours are run by club members, consequently the price is pretty cheap. They have simple 'down and back' tours to Cabo where you can drop out or rejoin any time, to 60 day tours of mainland Mexico. They also have a tour to Central America.
Then there is the travel club
Discover Baja. Here again they offer tours led by experienced members. Their 'Tour' page doesn't seem to be working right now (probably because it's not tour season). But they do offer short trips (day/2 day), to Guadalupe Valley Wine Country and Ensenada during the year. There is a winery day trip (non-RV) leaving on May 10.
Both of these sites are also good for insurance. You can get on-line quotes, buy your insurance, and print out your policies; all from your laptop.
The Tour Season for these clubs is the winter months and are mostly over by April.
Another option would be to join up with someone that has been to Mexico before. The ontheroad website mentioned in a previous post also has a 'buddy' page where you can find a buddy to team up with.
Finally, there are places (WalMart parking lots, etc) near almost all of the border crossings where people stop when looking for a buddy.
Hope this helps