I would recommend I90 to Butte. I15 to Las Vegas. Us93 to Kingman.
I40 West to Arizona 95. Arizona 95 south to Quartzsite. Us 95 to Yuma.
Salt Lake traffic is not all that bad if you avoid Fridays and afternoons after 3 PM.
If you are sure there will be no weather then US 93 all the way to Kingman would be OK. Then on to Yuma as I described above. Be careful as you can have ice covered roads in Nevada during the winter even when the weather is clear and sunny.
But if there is weather I prefer the Interstates.