My Brother lived at the RV park Donna listed, it seemed OK and safe. Nothing spectacular, just a big RV park. Kind green tho.
Check a map, there are a variety of ways to get to Oaks park from there, both Freeway and back roads.
This is probably the closest you will get to the Oaks. Just not much RV space in Portland proper.
PS.. I learned to skate in that rink as a kid, and later on, as an adult, I put in a huge sound system there. (Which, I am sure, has been trashed and replaced by now)