I haven't found a single good boondocking spot for big rigs on the coast. Very, very difficult.
In OR you'll find plenty of great State Parks and even some NFS campgrounds that can take a bigger size. Some of them do have dry-camping sites. This is your best bet for easy, coast-accessible big rig spots. There are even a few "pure" dry-camping possibilities in OR (e.g. the harbor at Winchester Bay comes to mind...right on the water for $14/night), plus several free overnight spots like some of the Casino's (e.g. The Mill Casino in Coos Bay or Three Rivers Casino in Florence)
In CA it gets more limited. Overnighting by the road is prohibited (at least all the places we've been) and even the State Parks are tight in many spots. There are options, like others have mentioned, but free boondocking is not really one of them.