"Arctic packages" usually just mean that the RV is (better) equipped to be used in sub-freezing temperatures, not that it can be stored in sub-freezing temperatures without external power or heat or whatever. Without this package or some equivalent, you likely couldn't reasonably be camping in winter with the water system active. Of course, what's standard and what's a separate package (and what's a "mandatory option") varies from maker to maker and model to model.
I don't think there's any RV made that's insulated well enough to sit in freezing winter weather for a couple weeks with no heat and not get below freezing on the inside. Many (probably most) houses, even modern well-insulated ones, would likewise not be able to survive that treatment.