I-10 to I-8 to Yuma AZ , then route from there to Vegas. With this you will avoid the horrible Phoenix traffic. You will go around Tucson and north a little to I-8 and as long as you don't go thru at rush hour it's okay. You also will be in warmer areas and flatter areas. You do climb in elevation from ElPaso TX but nothing you even notice and then you will go back down in elevation to warm temps. (hopefully - they are colder in AZ right now). We stop in VanHorn TX which puts you about an hour away from ElPaso. That allows you to time it to go thru after morning rush hour. Enjoy your travels.
