I would suggest you head south from Denver to Laredo but at Laredo you cross over on the Columbia bridge bypassing Nuevo Laredo and shooting straight for Monterrey. Take the periferico around Monterrey, head for Saltillo but just as you exit the Saltillo Canyon at Ramos Arizpe you swing left on the couta to Matahuala and Mexico City. First 40 or so kilometers a very heavy and taxing upgrade, but a forced and enforced truck route around Saltillo. And it is two lane in your direction. Down the main trucking highway from the border to Mexico City (wide lanes, good surface, heavy truck traffic) but easy driving and boring, bypassing San Luis Potosí and then at Querétaro the junction of the Mexico city-Guadalarara cuota you swing right to Salamanca and there swing left (south) on Mexico 43, a straight and mainly level shot directly to Morelia on two lane but well surfaced and not heavily trafficked highway. This route will give you excellent time and minimize the cuotas charged. If you haven't traavelled in Mexico with a RV you will find the coutas high, but usually worth it in time and nerves saved as well as getting you away from most heavy truck traffic on dangerous two lane surfaces.
If that heavy upgrade worries you, at Ramos Arizpe you can continue on toward Saltillo and take the exit to Torreón. From there to Fresnillo to Aguascalientes (bypassing the grades at Zacatecas) to León to Salamanca and then at Salamanca swing right onto Mexico 43. This route will involve a confusing and hour-long ride on the Aguascalientes periferico as well as considerably more cuota fees, but escapes really heavy grades and a lot of the heavy truck traffic.
You didn't say if your rig is diesel or gasoline. Gas was about $2.22 a gal in April. There has been a slight increase this month as mandated for every month.
Happy travelling. Adios. jerezano.
(This post was edited by jerezano on May 1, 2006, 8:26 AM)