Western trip - Olympic Pennisula

I would not assume that the NP campsites are all booked, also reservations, or arriving in the early afternoon would be a requirement.
No reservations, during the most popular period would require early arrival, flexibility and some luck

You want to see the Hoh Rainforest, the coast it-self, Hurricane Ridge and the Straights of Juan de Fuca

IMHO the Dungeness Spit is a must see. Try looking a the below for what to see on your road trip of the Oly Penn:
http://travel.nationalgeographic.com/travel/road-trips/olympic-peninsula-washington-road-trip/

If the description of your rig says you have a 26 foot rig, try Kala Loch(pronounced clay lock) campground. You might have to find a separate spot for the truck as the sites are in general small. It is a great base of ops for the west side.

North a bit is Palms where the latest vampire series was shot.

Just north nd I mean just north of the Kala Loch campground is a jasper collecting are. There areocean tumbled pigeon blood specimens to be found after high tides and storms.

Thar's all off hand, DW might add some stuff later.

I forgot about a St Pk to the west of Pt Angelus. Vest nice and quiet if available. I'll ask DW the name.

http://www.clallam.net/Parks/SaltCreek.html
Salt Creek is a county park on the shore west of PA.

There's also a county park with camping at the base of Dungeness Spit.
http://www.clallam.net/Parks/Dungeness.html

FRom Port Angeles, walk on the Coho for a day trip to Victoria. It goes from downtown to downtown. Victoria is a lovely city to walk around.

There are some wonderful state parks around Port Townsend - Fort Worden, beachsites are great. The area is an easy drive to Hurricane Ridge for a day trip. Other places to look are in Sequim and Salt Creek Rec. Area just west of Port Angeles. For touring the far western part, staying around Forks is a good base and there are fewer campgrounds on that side. It would be a lot of driving if you're trying to do it all from one spot. Have fun.

If anything it's the state parks that will be full, especially on summer weekends.

I didn't see any questions about getting to Washington. I90 and US2 both get lots of discussion. But once in Washington, WA20 through North Cascades Nat Park is the best route across the Cascades. And it takes you right through state-park alley on Whidbey Island and over to Port Townsend.

The main trick to camping the summer without reservations is to settle in early on the busy weekends, especially if you are a few hours drive from a big city.

Thanks for the responses from all! North Cascades sounds great, but is a little north of where I was thinking. How is US-12 from Yakima over to 101? That looks interesting...and avoids the Seattle metro-plex.

Hwy 12 from Yakima to 101 is a great way to go.

You could also stop along Hwy 12 near Packwood for a night and take 123 north to Mt. Rainier Nat'l Park (or check out it's campground from that end - not sure of the length of sites)

You could also take the road from Randle - 131 south I believe - which will take you to the back side of Mt. St. Helens Nat'l Volcanic Monument. Quite a view! Iron Creek National Forest campground is not far off Hwy 12 and is very nice to explore Mt. St. Helens. Don't take your RV beyond Iron Creek. We stayed here with our 40' MH and also used it as a base for Mt. Rainier with our towed vehicle. I believe it accepts reservations, also.

US12 is a spectacular route when heading west toward and along south flank of Mt. Rainier. Packwood and Randle are good places to stop to spend a few days to explore Rainier, Mt. Adams and backside of St. Helens. US 12 from there to Aberdeen is a beautiful and doable day drive where you pick up US101 for a 3-6 day trip around the Olympic Peninsula. Others have already given you some good ideas once you get there. It is a huge NP.