Why limit yourself to state parks? There's gotta be lots of private CGs within striking distance from Portland. We're Thousand Trails members and stayed at their Mt. Hood CG this summer (Welches) and it is one of the nicest CGs we've ever been to anywhere. It's an Encore park though and have to pay something like $60 a night but for the occasional winter camping weekend isn't that bad. They have some CGs over on the coast of course and there's Chehalis up an hour or so in WA which is a nice CG. Mt. Hood and Chehalis appear to be open year round. You can pay retail rates, get an annual pass for +/- $500 or find a pre-owned membership on CL for under $1K.
I totally love the drive up hwy 26 to Mt. Hood and can't wait to go to the T/T CG in Welches again. FWIW, you can buy an annual pass for Encore parks for $199 and stay in them for a lot less.
We camp in Blaine WA a lot (in a T/T CG) and it's open year round. Lots of CGs up around here near the coast that don't close over the winter. We camped in Blaine a couple of years ago in Dec. and although dry and sunny (and cold), there was hardly anyone there and it seemed like a ghost town compared to the summer. Very odd feeling...