As others have said - it's not the water tank or waste tanks that are typically the problem in moderate climates like Portland -- it's the small water lines and things like the toilet valve that tend to bust if the weather gets into the 20's. I usually blow the lines and put some RV antifreeze into the P traps of the kitchen, bathroom, and shower.
For what's it' worth -- I am not a fan of "storm watching" in an RV - hotels are cheap in the winter and if your there to watch a big storm that's where you want to be not some RV.
