May can be quite variable on the Oregon coast--lots of rain and miserable, or pretty mild. I'd plan on rain at least half the days of your tour (although many of those days will have lots of rainless moments). Bring a second set of rain gear and consider finding a cheap motel to take a break from camping and to dry out your gear. Daytime temps will probably be low-40s to mid-60s, but could reach mid-30s or mid-70s.