Hi Amanda,
I would recommend summertime! The northern part of the state can be cold and rainy in the spring and fall.
It IS a most beautiful drive though! Northern Ca, especially the Big Sur and Carmel area is awesome, as Hwy One hugs the coast!! The coast Hwy WILL take longer to get to San Fransisco then the inland interstate 5 but DEFINITELY worth the extra time!
5 is rather dull!!!!
San Fransisco is one of the most awesome cities in the world!! So fun to visit!!! I recommend walking everywhere and taking cable cars once you park the motorhome. Parking is not easy to find in the city. And do plan on staying there a few days atleast, lots and lots to see!!
The sequoia's/ Kings canyon are beautiful......but if you only have time for one I would recommend Yosemite!!
My husband and I camped in the Northern redwoods in April.....don't know WHAT we were thinking!! Beautiful scenery, Yes, but we froze!!!!
Santa Cruz is a fun place to take your kids...a fun board walk with rides, food, games, etc. Also a great Aquarium in Monterey.
The california coast is truly beautiful and...whenever you go...I hope you have a memorable trip!!
Hugs, Donna