Don't think you will have any problems finding a camping spot in the OR SP's in May, unless it's on a weekend, therefore I wouldn't bother with reservations. You're beating the heavy use time which is after school lets out. All OR SP's are fine, good showers and nicely laid out sites. One of our favorites is Harris Beach immediately north of Brookings OR. Buy your fuel in OR, a lot cheaper than CA where you'll be coming from. Take US 101 all the way up to Astoria, then cut over to I-5 thru WA, avoid the Portland area traffic during commute hours if you're in that area.