Wow.... not a lot right around there. The closest thing I can think of is at the other end of 217 and then a few miles down I-5 in Wilsonville.
http://www.pheasantridge.com/
It's probably +/- 20 minutes from Cedar Hills during light traffic times but could be 45mins-1hr under bad circumstances (yes, traffic around here is TERRIBLE)
Here's a super nice state park towards the coast from there:
http://www.oregonstateparks.org/index.cfm?do=parkPage.dsp_parkPage&parkId=75
Probably a 45 minute drive though.
That's a tough area for RV parks/hookups.