Cross into the US at Sweetgrass, Montana / Coutts, Alberta
Follow Interstae 15 south to Helena, MT
In Helena take US 287 south to Interstate 90 East
Follow I-90 East to the Buffalo, Wyoming and then take I-25 South
Follow I-25 into New Mexico.
This is the route with the lowest elevations and keeps you on the east side of the Rocky Mountains. Depending on when you leave you will most likely have freezing temps at night during much of the trip.
Here is a relief map of the US showing elevations.
http://www.learnnc.org/lp/media/maps/usa_toporelief_1968_l.jpg
Alfred May
2005 Excursion V10 4.30 4x4
2002 Cedar Creek 30RBS TT by Forest River
Reese Dual Cam
Tekonsha Prodigy