From Seattle, I would go across to the Olympic Penn., try Kala Loch over on the Ocean. Nice drive into the Hoh Rain Forest, see the glacial river there that is blue grey from the rock flower. Hikes for the kids. There is a Washington St Park E/O Sequim Camp Ramblewood IIRC. There is Deception Pass St. Park on very North end of Whidbey Island.
Fort Stevens in OR., lots of folks but that is a popular time of year. Across the River back in Washington, Cape Disappointment for Lewis and Clark, E/O Fort Stevens is Fort Clatsop where the Corps of Discovery wintered over before heading back to DC.
Not in any order by mileage or importance or activities are Depot Bay, Otis Cafe in Otis, South Beach in Newport and the Aquarium and Hatfield Marine Science center. Another Marine Science Center in Charleston out of Coos Bay, Beverly Beach St Park, South Beach, Several towns in OR have public crabbing docks and you can rent gear, Waldport is one, there is McKinnnley'd Marina and RV part about a mile from the dock in Waldport. Our favorite right now is Winchester Bay RV Resort, lots to do around there, you can even try Camp Sparrow Road out to the Beach and drive on the sand. We took the Suzuki out there. We haven't camped S/O there because we have family in Florence, they have lots of sand dune stuff between Florence and Winchester Bay. Lots of things going on in Yachats all summer.
Check on line for both Washington and Oregon State Parks, they have maps and POIs they will send you absolutely free of charge, hoping you'll visit and have fun spending your money up there.
Oops, Florence and Winchester Bay have BJ's Ice Cream, don't skip that. Lakeside just S/O Winchester Bay has dry camping at the Fairgrounds IIRC. It's just off HWY 101. Crescent City has nice campground called Shoreline RV Park, go to crescentcity.org to make reservations. You can call but they prefer on line.
For Coastal Oregon, make sure you let them know you have a 30' TT. A lot of the coast parks have very few big sites.
I'm sort of bouncing around as I am trying to cover a lot of ground and one comment will jog my memory about something else, like the Samoa Cook House in Crescent City. really good breakfast, lunch and dinner, the Chart Room in CC is crowded on the off season so summer should be an hour or more but the seafood is worth it. The Rougue Brewery Public House is a good family place(a bar too)for hamburgers and their on site made potato chips.
If you need anymore help PM me and I'll see what we can come up with if you are specific about what the kids and you like to do.
David
"I travel not to go anywhere, but to go. I travel for travel's sake. The great affair is to go". R. L. Stevenson
David Bishop
2002 Winnebago Adventurer 32V
2009 GMC Canyon
Roadmaster 5000
BrakeBuddy Classic II