Some comments:

SF to Big Sur to Morro Bay is best seen by taking CA-1, not US-101. See if you can work in a Hearst Castle tour. I agree with Michael - instead of taking I-15, go through Joshua Tree and spend a night in 29 Palms. Then take Amboy Rd. and old 66 to meet up with I-40 near Needles. You can then head back to the old road up through Oatman and then on to Kingman - and stay on the old road to Seligman.

My only concern with touring Utah in December is the possibility of snow. To enter Zion from the east can be tricky - the road is very mountainous and winding, you have to go through a tunnel, and the road is very subject to icing conditions. You will need to keep track of the weather and conditions.

A better way into Death Valley from LV is the "locals route" - 160 to Pahrump, Bell Vista/Ash Meadows/Stateline roads to Death Valley Junction and in the east entrance on 190.