Have you considered the Pismo Beach area?
Perhaps I'm mistaken, but San Francisco doesn't seem like a "snowbird" area to me. The winter weather is usually pretty rainy and cool during the winter months (oh goodness, we are all hoping that it's going to be a more normal winter this year). The further south you go, the less rainy and warmer it is during the winter.
There are some nice RV parks in Pismo, but I don't know which of them do monthly rentals.
