Howdy. We have done it. Great trip but take your time in getting there through Mexico. Travelling through Mexico is one of the most rewarding RV trips you can make. Great country, super interesting history and culture, friendly people everywhere and well preserved architecture. I wouldn't say its any more or less expensive than RVing in the rest of North America as many of the highways are toll highways. Highways and interstates for the most part are very good and pleasant to drive. Somebody mentioned drug cartels. For the most part they won't affect you unless you are buying or selling drugs. They are too busy getting their product to the rest of North America. Your route will be determined by your starting point but I would recommend the Saltillo through Zacatecas route. Amazing cities. Did I mention the food? :)
Someone mentioned bribes. Can't help you with that. In all my miles and states visited in Mexico we have never been asked for a bribe nor do we know first hand of that happening. I'm sure it happens, We have dealt with cops a dozen or so times and all were helpful and professional. Just like violence, I'm sure it happens, we just have no first hand experience with that kind of thing. If you don't speak spanish you should have a phrase book with you. Make sure you have your basic "please and thank you's down.
Off topic slightly. Belize is okay to visit but wouldn't be my choice for destination. The Mexican economy is in high gear right now and has one of the fastest growing middle classes in the world. Not so much in Belize. If you need major medical you will be travelling to Mexico anyway. Mexico' per capita income is about 4 times higher than Belize and it shows. Don't get me wrong, we like Belize but it is a third world country unlike Mexico. There are some nice areas on the peninsula right close to the Belize border that could make for a long term stay and there are already some expat communities in that area.
Whatever you choose I wish you luck. It is a great trip and a lot of fun. I'm sure either country will suit you if you find the right place.
One other thing, I'm sure other posters on the forum are well meaning but I would recommend you listen to those who routinely travel in Mexico. Nothing wrong with reading the news and travel warnings but keep it all real. Just to put it into perspective, the Capitol of the USA Washington DC has a murder rate 9 times higher than the Capital of Mexico, Mexico City...and yet we will be visiting it in two years. Travel smart and be aware.
Happy trails.
John and Angela