In my opinion, I-10 or I-40 would be your best route choices going west. You will get to some altitude levels but the roads will be nothing like pulling through the Rockies. I pulled a UHaul back to Amarillo from San Francisco running about 70 over 2 1/2 days and burnt the valves out in my old van. If I were to make that trip from where I live now, I'd plan on a week both directions, plus the time I would want to be out there. But that is just me.
'99 Ford F-250 SD and Vengeance 29V Toy Hauler
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2015 Forest River Vengance 29V Toy Hauler
1999 F-250 CC with 5.4L (under powered but it is paid for
2002 BMW K1200LT for the TH garage