I'd try to find a camping spot in the Suislaw National Forest
Siuslaw National Forest - Camping
(Highway 34 from Corvallis) which will put you in a great location on the coast and would give you some options on the way to get to Crater Lake. I assume you are driving to Oregon, not flying? Depending on your means of transportation and what direction you need to go to return home, you may not want to spend the night as far east as Crater Lake. You could camp in the Roseburg area (Cavitt Creek Falls, for example) and take an easy day trip to Crater Lake from there
Recreation in Roseburg, Oregon
That way, if you need to return to Corvallis or get to Portland, you'd be very close to I-5 to get there. Also, try to make a reservation for camping if you can.