We did the west coast trip beginning the last week in February 2015. We went from Virginia to LA and back. Returned in April.
I had been planning that trip from before I retired. There were some absolute Bucket List items for me, and believe it or not, downtown LA was one of them. Death Valley NP was another one. The Alamo was also on that list.
Once I got the big points settled I looked at the various routes to do that and what other national parts, etc would fall along or near the chosen routes. Then I'd add those to my list of places to stop.
As other's have mentioned, take your time. I tried to limit my daily runs to 300 to 350 miles. There were a few times when I stretched it, because there weren't any reasonable places to stop before hand, or it was logical to stop. For example, if it were 400 miles between points of interest I'd just drive the full 400 miles.
Also, I recommend getting off the interstates as much as possible. I suspect 70% of my drive was on the blue highways. It's a much better way to travel, more relaxing, and much more to see. This especially true once you cross the Mississippi. Even when we were crossing Mississippi along I-20, we'd jump off and drive the blue highway that mostly paralleled I-20.
In LA there had been things I'd wanted to see since childhood/teen years, such as the La Brea Tar Pits, Mullholland Drive, tour a major studio, etc. So we stayed in LA for 6 days. Stayed at Dockwieller Campground on the beach in LA for 2 nights and at Balboa RV Park at Van Nuys for 4 nights.
PM me if you would like any more information.