There is some great information about Mexican insurance on this forum, but there a multitude of options on how to best insure your RV in Mexico. I highly recommend calling a Mexico insurance specialist to explain all of your options because RVs (especially when you are towing other vehicles) can be a bit complicated to insure.
I have had very good experience with Adventure Mexican Insurance
http://www.mexadventure.com 800-485-4075 They take time to explain all of your options, and the pros and cons to each type of insurance.
There are a lot of options depending on what type of insurance you have in the US/Canada, the length of time you will be in Mexico, and if you plan on going to Mexico frequently.
Option 1 - switch your US insurance carrier:
If you plan on being in Mexico for more than 7 days, you should highly consider switching your US insurance to a US RV insurance company who extends physical damage and theft coverage into Mexico (there are about 3 US companies who offer this perk right now). This option can save you hunderds $, and can give you much better coverage. These companies cover all of Mexico, not just 25 miles across the border like some other US insurance companies. If you have this special US RV insurance, then you will only need to buy a Mexican liability insurance policy which is much less expensive than buying a full coverage RV insurance policy (plus you get lower deductibles and often get full replacement value through your US company - the Mexico insurance only offers depreciated values and usually has higher deductibles). NOTE: Canadians, sorry, this is not an option for you at this time.
Option 2 - If you are Canadian or do not want to switch your current US insurance carrier:
If you need to get your 'full coverage' insurance through a Mexican carrier, you should ask the following questions before your buy:
1: What are the deductibles?
2: Am I allowed to fix my vehicle in the US, and what is the hourly labor rate for repairs made in the US? (many companies make you fix your vehicle in Mexico - yipes!)
3: Is towing included?
4: Is legal assistance included?
5: Is Medical Evacuation and Plane Tickets home included? If so, for how many people? (We found that Adventure Mexican Insurance includes medical air ambulance and plane tickets back to your home if your vehicle is not drivable - this coverage is already included in their Mexican policy for up to 4 people in your tavel group)
If you are towing another vehicle:
Each self propelled vehicle will need its own individual Mexican insurance policy for while that vehicle is driving under its own power. So expect to buy 2 separate insurance policies (but you will also need to list the towed car as a towed unit on the RV policy).
As you can see, this gets a little complicated, so I highly recommend speaking to a live human who can explain all of your options to you.