I hate to be the bearer of bad news but RV storage in the Portland area is very (!) limited.
We were storing a unit at a facility but the owner came out and said he could no longer allow RV's in the lot (long story) and we (plus many others) had to move. Fortunately I was able to work things out with our neighbor and park next to our house.
Called around and couldn't find anything. RV sales are booming in the Portland area but no new storage facilities have been built.
The only one we found that seems to have at least some turnover is Chinook RV Storage (ChinookRVStorage.com). The pictures are a bit misleading as they have very limited undercover storage (nothing available) but about 250 spaces out in the open.
Good Luck with your search.
Due to depreciation the above opinions are now worth less than what you paid for them.
We are all good at something.....
I just happen to be a good bad example.
2016 Forest River Forester 3011DSF