I lived in Portland Metro for 33 years and bought my first Class A in '04. Then in '06 I traveled up to Alaska and ended up spending 5 summers and 3 winters in Fairbanks. So I had some experience living in my RV at 20°F for weeks.
I would, if I was doing this in Portland, add a shut off to the outlet of the freshwater tank, that way I could leave that tank full while being able to drain the small supply pipes. And I'd just remove the water pump, drain it, then put it back unconnected. (On my Bounder I could unscrew the couplings on the pump by hand, and it was easily accessible).
Your '08 likely has the newer PEX plumbing, (it's milky white) which is resistant to freezing. So a good draining at the lowest point in the system might be good enough. Go around the RV from the highest water point in the RV (usually the ice maker supply) and open the valves or couplings. Wouldn't hurt to also blow them out out with air. Then leave all valves open. Also pour pink stuff into all the drains. Automotive stuff weakens the seals at the tanks dump valves.
Also, you'd want to drain the water from your hot water tank. And remember to drain the inside/outside showers.
