The missus and I are planning a similar ride this fall. We are going to do the staging thing. The plan is to ship the bike to Las Vegas, fly out and ride it back. We are planning two weeks and even riding just one way it seems hard to fit in all there is to see.
I would reccomend against taking the freeway. The secondary roads through that area are such that you can still make pretty good time. The freeways are just too mind numbing for me.
2005 BMW F650GS
2012 Triumph Tiger 800xc