I did a trip I called '7 passes' where we drove over the 7 sierra passes from Yosemite up to the Oregon border. Plan for after June to be sure the passes are open. We 'camped' in ski resort parking lots, but usually not too hard to find a campground if you pre search where they are located.
If you like 'wine country', don't forget the Shenandoah Valley area along Hwy 49 and E16, South of Placerville. It's much less crowded than the Napa Valley and the wines are better! Ha!
Placerville, AKA Hangtown and the surrounding areas like Coloma, are historically interesting. In fact, a lot of the towns along Hwy 49 are historical and have lots to see. Lots of small areas off the main line of 49 as well, such as Fiddletown, Pokerville Flats, Jackson, even Folsom and the prison are all interesting sights to see.
Look up a book called Northern California Back Roads to get ideas of what there is to see off the main routes. Most roads are in good to excellent shape as a lot of them have been resurfaced over the past 5 years or so.
Oh, Bodie ghost town is interesting, but the last few miles in is dirt/gravel road. There's a parking lot to drop the trailer if you wish on the way in off 395.
Enjoy retirement, sounds like you are getting off on a good foot!
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I'll add stuff as I remember them.
Skunk Train
Roaring Camp and train ride.
Shelter Cove if you really want to get out there along the 'Lost Coast' and not get into too much twisty. Nice camp ground there and the fishing is great too! Lots of whale watching.
Redwoods are impressive and there are lots of places to see those on your route.
Winchester Mystery House
The Ronald Regan Presidential Library