I did a trip in November that was in many ways similar to yours, and I can echo what everyone else has said. The original itinerary was simply way too packed. I very nearly made this mistake myself, and was talked out of it by several people here on FT. I'm very glad I was, because otherwise I would just have been racing around ticking boxes.
My trip consisted of San Francisco, Yosemite, Mammoth Lakes, Death Valley, Las Vegas, Zion, Bryce Canyon, Page, Grand Canyon, Flagstaff, Winslow, Phoenix and Tucson.
That itinerary took me 27 days,
without wife and kids, and I felt I had nowhere near enough time in the absolutely stunning Zion and Bryce Canyon (my fault for trying to rush and ending up with a few dead days at the end).
If you want to actually
see anything (other than through a window at 75mph), you need some multi-night stops.
San Francisco is an enjoyable city - I spent 4 nights there, including trips to Alcatraz, Marin County and Muir Woods - all worthwhile.
Yosemite is definitely worth two nights, minimum- I happily strolled around for about 7 hours there one day.
Mammoth Lakes is nice, but I was content with a lunch stop and a one-hour walk. Probably a good place to stop on a long drive and let the kids burn off some energy.
I enjoyed Death Valley, but I was there in November. Having toured Australia in Summer, I just can't imagine Death Valley in July. You will spend the whole time stuck in the car with air-con blasting. Either that, or you will just in some nondescript cafeteria somewhere drinking iced coffee.
Zion National Park and Bryce Canyon were both simply stunning. These two days were among the highlights of my trip. I could very easily have spent a week in this area.
Vegas was worthwhile for the experience - but then I was doing the trip solo
- I'm not sure whether you would want to spend any length of time there with the wife and kids.
The Grand Canyon is a must-see. How long you spend there depends on how much you like walking. I filled two days just walking the trails at the top.
The best advice I can give you is:
1. Don't try to pack too much in. You need some chill-out time, and you need to take time to appreciate the things you see.
2. Don't be afraid to switch your planned itineraries around. Throw different ideas around and see what works for you. The trip I eventually took was quite different from the one I planned.
Here is a link to the planning phase of my trip - maybe there will be some useful ideas in there for you too:
http://www.flyertalk.com/forum/west/1744380-western-usa-road-trip-advice-please-3.html
Also, beware of the problems on Ca-Hwy 1 this year. There is severe disruption due to landslides and a damaged bridge, which has to be demolished. I was planning to drive this route this fall, and have more or less shelved my plans:
http://www.flyertalk.com/forum/california/1585849-pacific-coast-highway-ca-route-1-between-sf-la-closed-indefinitely-near-big-sur-19.html#post27956984