Wait... are you buying in Oregon and then parking it on a permanent site in California? If so I would just get a temp tag from the dealer when you buy it. No sense paying yearly registration fees for a vehicle that doesn't move. Again, I'm not familiar with Cali requirements. In Maryland, you don't have to have tags unless its moving down the road.
Yes, you pay sales tax on it. I'm not sure how taxes will work in Oregon/Cali in your scenario. Usually you pay sales tax when you buy (from a dealer), and they send that money to the state where you register the vehicle. In your case, if you don't have to register the vehicle, I'm not sure how that would work...
Insurance is insurance. My truck's insurance covers the trailer when I'm driving. My RV insurance covers the trailer when its parked. Now, whether they will let you only buy 1 months worth of insurance is another question.
In the end, it begs the question... why not just buy in California and have it delivered?