raston,
I can't help with the RV parks. However, you will find the
Oregon State Parks to be outstanding....and likely less expensive too.
Yes, in the summer reservations are hard to get, but if you arrive early in the mornings, you are likely to find some openings. If not, just have an alternative in mind. unfortunately, there are only a couple places on the coast where you can boondock. But, if you drive inland a few miles, you will be in the coast range where there are numerous Forest Service C.G.'s. Not on the coast, but at least a place to stay.
On your way back, I suggest hwy 199 to Grants Pass, then hwys 62 and 138 over the Cascades (you could swing through Crater Lake N.P. for a day or two), then take Hwy 97 north as far as you want. Check out Central Oregon (northern part of Cascades Lakes Hwy and Smith Rocks State Park) if you have time. The scenery will be vastly different than the coast.
Have fun,
Steve
Oh yea, that early in the season the Cascades will have mosquitoes so be prepared.