Some RV manufacturers have an optional winter package available. Make sure whatever you buy has this, as a minimum.
Just as a reference: Our Winnebago 24V came with the winter package option. This includes two coach batteries and 12 volt electrically heated grey and black tanks. Of course these heaters don't drain the batteries when on hookups because then the stock converter powers them, but being 12 volt heaters they can also be used for drycamping in the extreme cold. All fresh water plumbing and the main freshwater tank are contained within the RV's heated coach environment. We also pack along insulated window covers for every window at all times, just in case we intentionally or by accident find ourselves camping in the extreme cold.