We would suggest that you head the shortest route and directly to your "need to be" California destination. Then you can more accurately judge how long a drive it is and how many hours per day you feel comfortable. We would also suggest that you not make reservations. There are plenty of RV parks in the area.
For your return trip, do take the Yellowstone route if you really want to see it. Again, there are plenty of sites not allowing reservations within the park. If you stay nearby the previous night and get into the park by 9-10am, you'll get a site.
We travel to those places constantly as full-timers and we rarely make reservations. We've been to Yellowstone many, many times and have never made reservations. You'll have a more relaxing trip if you 'wing it'!! Have a good one.
