The OP has a 40' motorhome so I doubt he'll be using many of California's state parks. California is a different country. :)
It's an altogether different story for Oregon. There are many state parks in Oregon for a 40' motorhome. We've been there with our 40'. We've volunteered in the state parks. Pull in early morning and in May you'll get a site without reservations. There are also plenty of private parks in Oregon. You won't have a hard time finding a site - even without reservations. In Oregon there are also some nice casino parks and county parks.
When folks make the statement that "they're all booked up for 7 months" they probably don't realize that the parks don't book every single site. There are always ones available just by driving in. This goes for the national parks in our country, too. They don't book every site. I just cringe when I hear things like "you'll never get a site" thereby deflating the person before they even try. This is so untrue. We rarely make reservations with our 40' motorhome, even for the major national parks. We get there early morning and seem to get a site easily. Traveling with reservations really limits any kind of flexibility.