Portland Metro area boondocking's a tough one, especially on the west side. Seems like development is creeping farther and farther out towards the ocean!
If you were going after Memorial Day I'd recommend the BLM campgrounds on the Nestucca River out of Carlton (
link), but since you'll be there before they open:
There's a Walmart in Mcminnville...overnighting policy unknown to this camper.
But this time of year, if I were headed back to the Coast from Tigard, I'd overnight at the Baldock rest area at I-5 sb exit 281. It may look out of the way by a bit, but there's an easy route through Wilsonville back to 99 and thence to the Coast. As you can see from the aerial photo
at this link, there's a big area at the back where RV's can park right next to a wooded picnic ground. I've spent many a night there.
The rest stop itself has a unique history- it's one of the biggest in Oregon and is the location of an effort in the sixties to plant a tree from each of the fifty States. I forget how many trees have survived, but I understand there are efforts underway to restore the grove to the full fifty.
The Grove of The States