Simple basic instruction from Laredo to Mexico City – 85/85D from south of Laredo to Monterrey – then 57/57D from near Saltillo to Mexico City. Roads with the D following the number are toll roads and those without the D are free roads. You need a very good map – or better yet a very good GPS.
To go to the Yucatan by going toward Mexico City but avoiding Mexico City traffic there is a recently completed road called the Arco Norte (40D). They have their own website where you can get and idea of where it goes and what tolls might be . . . http://www.arconorte.com.mx/
If going toward the Yucatan you would then take MX 150D from Puebla over the mountains and then MX 145D to MX 180D on to the Yucatan. These are all toll roads similar to US Interstates – they are called Cuotas in Mexico. Or you could take the free roads called Libres – but roads can be rough in some areas and going can be slow – though with experience we learn which Libres we can take with few problems.
The following website is not perfect and can be confusing but with a little practice it can be figured out. It will tell you where the toll roads are and how much they cost. The trick with an RV is figuring if you are two vehicles. And some roads charge for dual wheel axles I am told but I am never sure. If you have a motorhome towing a car you must find the toll for a bus and for a car and add them together – though this is not always true of all roads. If you have a pickup towing a trailer you must figure the toll for each of those and add them together. That will give you something of an estimate though it may not always be exact. Experience is the only way to be able to better estimate costs. . . . . .
http://aplicaciones4.sct.gob.mx/sibuac_internet/ControllerUI?action=cmdEscogeRuta&eligeIdioma=si