I'd say the single most important piece of advice I can give you and anyone else who reads this thread in the future is this: The Peninsula is close enough to the population centers on the I-5 corridor to fill up on nice weekends in the summer. Have a reservation or plan to get here Thursday/early Friday morning or you might not find a campsite at all. Arriving midweek is usually ok unless there is a major holiday.
By the way, I forgot the two Forest Service campgrounds in the Forks area.
This especially true for Kalaloch. I speak from bitter experience.
Neal & Sharon
A good traveler has no fixed plans, and is not intent on arriving.