Driving to Belize

Depending on where you are in BC you have a few options through the States to the Mexican border.  Between the I-84, I-84 and US 285 or the I-90, there is not much difference distance and time wise. All 3 routes end up in Del Rio, which is where we crossed. Small crossing. Easy to get to on the US side, and very easy to get through Ciudad Acuna, the town on the Mexican side.
From Ciudad Acuna we took the 57 to Saltillo, San Luis Potosi, Santiago de Queretaro. The Arco Del Norte takes you just north of Mexico City towards Puebla on the 85. The 85 will join the 150 to Cordoba. Stay on the 150 until splits. There you'll take the 180 to Minatitlan and Villahermosa. From there it's the 186 all the way to Escarcega and Chetumal. These are mostly toll roads and usually have a D beside the highway numbers. The non toll roads usually run pretty close to the toll roads so if you miss an entry/exit, it's not difficult to find your way back onto a toll road. Good maps will easily identify the toll roads. Google maps are pretty good as well.

I have not done the drive but I know lots and lots of people have and there are people that make their living driving people down there the one pointer I can give is cross the border first thing in the morning so that you can get as far south as possible over the course of the day and do not drive at night

http://www.mexconnect.com/

We drove through Mexico from California in our 26-foot RV and had a great time (other than the awful roads). The RV parks were inexpensive and we parked in hotel parking lots when we couldn't find a park. Mexico is super inexpensive and the people are warm and welcoming.

Our route took us from the Arizona border crossing, down the west coast to about Guadalajara, cross country to Veracruz, and then down to Chetumal and into Belize. It took us 2 months because we were in no hurry.

If we had it to do over again, we'd reroute to avoid the big cities and the extra awful roads in south Veracruz. We'd be happy to answer additional questions if our route sounds remotely appealing to you.