If you worry about it raining in Oregon; IT WILL.
Have you been in Oregon at all?
A loop is what I would do. Go north up 97 to Bend and then west on 20 to Sisters, Oregon and 22 to Salem to Lincoln City on 18. You could go North on Oregon 101 to Astoria, but that should be another trip.
Most of the time it will be dry on the east side of the Cascades (Bend area).
Heading south at Lincoln City could be your turn around point to head towards home via Oregon 101. South on 101 is far safer. The pull-outs on on the ocean side, but only a few are big enough for big rigs.
If you let us know what you want to see and do, we might be able to help a bit more.
I won't camp on the Oregon Coast after mid May. There is just too many tourists for me. I travel over there almost every week as it is for work (Florence to Lincoln City is my service route area).