While Oregon and Washington state parks are usually great, there are lots and lots of great private ones along the Or. and Wa. coast too. We stayed at Secret Camp RV Park once, next to the Rogue River at Gold Beach, and it was wonderful (20 sites only). The owners said that the get the same repeat visitors from Europe every year because they like it so much. If into horse riding, you can do that on the beach at Pacific City which is fun. There is a Thousand Trails there that is really nice.
A visit to Seaside is recommended and there are a number of CGs in the area. The Fort Stevens military museum there is interesting and worth a visit.
Just across Columbia river in Wa., there is Cape Disappointment state park in Wa. with RV sites close to the ocean. We drove around and it didn't really excite us much so we went to the TT just up the road a bit which was nice. Farther up the Wa. coast, there are lots of private CGs around Ocean Shores. I'm not sure about Oregon, but in Wa., much of the foreshore is government owned and you won't find many CGs right at the beach. We stayed at Ocean Mist RV park at Ocean Shores and the beach was 500' or more away and you could only hear the waves. The wind blew hard non-stop and was so loud you could barely talk outside. We walked way out to the water at low tide one day when it was really foggy - when we turned around, we could not see the shoreline at all. Footsteps in the sand were the only thing that got us back. Fun experience there.
Gotta drive around the Olympic Peninsula. There aren't a whole lot of CGs there though and I assume you need to book well ahead. Then I would drive all the way 'round and then down to Port Townsend. It's an easy 1/2 day or so drive all the way around from Ocean Shores to Port Townsend. There is RV camping near Port Townsend and we have stayed at Diamond Point Resort which is small but nice. If you drive to Port Townsend, there is a ferry ride over to Whidbey Island which is worth doing. The drive up the Island to near Anacortes is nice, which will then take you over to the I5. The state park at Birch Bay is quite nice if you want to keep going north but it is very popular.
You might want to consider getting a Thousand Trails membership. They have a sale on at the moment for $445 which would get you a 1 zone pass that would cover Wa. and Or. All the ones I've seen so far are on the rustic side with lots of trees. Haven't seen a bad TT CG yet. A Coast-Coast membership or one of the others would get you a discount at some private CGs.
If heading along the Wa. and Or. interstate, our experience has been that there aren't a whole lot of CGs to choose from like on the coast, esp. in Or. We have stayed twice at Howard Prairie Lake Resort east of Medford twice now and thoroughly enjoyed it. In nearby Ashland, there is the annual Oregon Shakespeare festival which I believe runs all summer.
If you make it as far as the Canada/US border and still want to get even more out of the already awesome PNW adventure by that point, you could take the ferry from Port Angeles over to Victoria on Vancouver Island. Then make your way up the Island and take the ferry from Comox over to Powell River on the Sunshine Coast, and from there, head down to Gibsons where there is another ferry to Horseshoe Bay in West Vancouver. There will be lots of CGs along this route. Be aware that BC Ferry rates are expensive and IIRC would be around $500 for a TV+TT. On the way back into the US, you could take the ferry over to Port Townsend and drive down the west side of Puget Sound and end up near Olympia which will avoid Seattle altogether.
This is a great book for RVers written by RVers, focuses mostly on non-private CGs and has detailed reviews of each CG:
