Hi pianotuna,
Please understand, living in the S.E., I normally park under the shade of trees where a/c is generally more important than heat. I can only relate my experience, having traveled on business to PA/Ohio/NY in the dead of winter in a camper w/2 GRP 27 batteries, LP 3600 generator, and no solar. -8°F is the lowest temp encountered, but perhaps my comments will help.
Campgrounds are closed, necessitating dry-camping. Between the furnace and generator usage, propane cost approaches the national debt. A 15 amp connection while parked at a customer's business location is very appreciated. I take on potable water at inside spigots from commercial auto bays, ie., Walmart, Goodyear, Firestone, and small entrepreneurs etc. I carry a small tote and use it like a cassette toilet at service stations with outside ingress to restrooms. While I wouldn't want to deal with that on a continuous basis during the winter, it gets me by for the 5-8 days on a trip to the frozen tundra. Seems I'm always traveling in heavily overcast skies on those trips. I'm interested to see how your solar experience turns out.