Since you're planning this so far in advance I'll recommend making reservations early! I'm putting together a semi-last minute trip to the Sierra...things would be a lot easier if I'd applied for wilderness permits and campground reservations months ago!
Sequoia and Kings NP
"Campground reservations for 2018 are available beginning December 1, 2017, up to six months in advance of your stay."
Inyo National Forest
"Standard campsites can be reserved up to six months in advance."
Yosemite NP
"Campground reservations are available in blocks of one month at a time, up to five months in advance, on the 15th of each month at 7 am Pacific time. Be aware that nearly all reservations for the months of May through September and for some other weekends are filled the first day they become available, usually within seconds or minutes after 7 am!"
(guess where I'm going)
Stanislaus NF
"The Stanislaus National Forest has 47 campgrounds which provide 1,514 campsites for visitors to enjoy! Most of these campgrounds on a first come - first served basis, some are avaialble on reseervation such as Spicer Group Campground, Big Meadow Group Campground, Pinecrest Campground, Pioneer Group Campground, Dimond O Campground, and Pines Group Campground. Check the individual campground pages for information on how to make reservations."
Redwood National and State Parks
"Reservations for the four campgrounds can be made through
Reserve California, the California State Park reservation system.
Reservations must be made at least 48-hours in advance and no sooner than 6-months before your anticipated camping date(s)."
Planning trips seemed a lot easier in WA, and I was always able to find first-come first-serve sites in OR, but I wouldn't be surprised if that's changing as the region grows. Wilderness permits for the Olympics are easy to secure w/o reservations and solitude is easy to find on the trails, but at the same time it's one of the most visited national parks in the country so the drive in campgrounds are probably booked thru peak season (I never car-camped the Olys so not speaking from experience).
Bah. Wish me luck getting my wilderness permit and finding a campground, looks like I'll need it!