From Northern Oregon, I would stay west of interstate 5. There's lots of good bowls along the way. Unless you decided to go East in the lower half of Oregon and hit up Crater lake. From I-5 go east on highway 138 to get to Crater Lake then work south on Highway 97. Klamath Falls has a decent park. Continuing south towards California, you would hit Weed. There's a funky park there that you can see the volcano Mt Shasta a few miles away. I-5 isn't far away. You could hit up Shasta City skatepark. Further south, I think Redding still has some trails going. Going further South, maybe hit up Anderson skatepark.
If I were giving my best advice, going south west from Grants Pass, Oregon on highway 199 towards Crescent City to go south along the coast on hwy 101 through the redwoods. That's one of my favorite parts of California. There is a little skatepark in Arcata. Lots of state parks through this part you could park an even at. You would be subject to go south on 101 to the San Francisco Bay Area. There's a handful of parks to hit along the way south. Petaluma, Novato, Saint Helena, and Napa have parks.
Since you're in the area if you go this way, hit up the Glory Hole. It's the overflow fullpipe at Lake Berryesa. There's campgrounds and I think even an rv park right downstream. Definitely this is something to definitely do. From there, you could go east to Sacramento. We have a couple good skateparks. If you do go to Sacramento, going south from there down hwy 99 there's a decent amount of small cities along the way with good skateparks. Elk Grove, Fresno, Delano are good. Maybe hit up Yosemite?
If you don't go to Sacramento from the Bay Area, I would skip San Fran. Go south on 680. There's a couple parks to check out in Concord and Walnut Creek. Further South San Jose has a really good park (Lake Cunningham) and a few more alright parks. South from there the Gilroy park could be worthwhile. Santa Cruz and Monterey are out of the way but neat for the tourist bit. There's probably trails in Santa Cruz. You'd have to go back east to get back on I-5 to get to LA. There's a fullpipe you could get to. It's a couple miles north of Patterson on the North bound side of the freeway.
Then you really can only go south and I can't think of much to do between there and LA. You should have it figured out there. However, if you did you the highway 99 route you could go to Woodward in Techapi.
Best of luck! Hit me up if you come to Sacramento or want some more details