Blowing out the lines may work for you. Prolonged periods of sub-freezing temps will freeze any standing water in the lines and traps. It sounds like Portland doesn't get that cold. The real issue with freeing water systems is not the piping, which can expand somewhat, but the fittings which just crack when frozen.
Be careful with the compressor. Set the regulator with the air flowing to 40 PSI to avoid over-pressure. Camping World has a fitting that goes from garden hose to Schrader.
Don't use auto anti-freeze anywhere. Use pink stuff or mineral oil in the traps.
