Yeah, there's usually a floor type vent or two that steals some air from inside and drops it down into the wet bays. If Winnie says yours is passive, than that means forced air from the furnace isn't being sent there. But there's an open vent somewhere and the cold air from inside the RV sinks down into the bays. Cold air sinks, but the cold air from inside the house will be probably 10-20F, or more, warmer than outside air.
Park your RV where the afternoon sun will hit the wet bays. Every day. Don't park in the shade, under cover, or where you can't be sideways to a southern exposure.
What I'd do is get some of that bubble insulation with the foil front and back. Cut custom pieces for the wet bays trying to insulate those walls and openings as best I could. While making them easily removed so I could dump occasionally. (I dump once every 1 to 2 weeks). And then I'd have a 100 Watt light bulb in each bay as well.
Also cut pieces of that bubble insulation for every window in the house.
Since you likely get snow, you might want to surround the RV with hay bales. And pile show up on top of them and against the RV where you can. Sheets of that foam insulation (that blue stuff) can be cut to insert into the wheel wells. And you can use it to block off the front windows, front and drivers and passenger's windows. If you don't need the front chairs, put it in there floor to ceiling behind them and watch a small TV in the rest of the space. Those forward entertainment cabinets are often under insulated. And the windows allow lots of cold to seep in.
It's best if you have lots of heating tape. Wrap around the drain valves. Wrap around the drain hose too. Leave the drain hose unconnected but capped at the house end, and stuffed into the sewer at the other. Only connect it when ready to dump.
Water is only used to fill your onboard tank. After that, drain the hose and store it in a heated area so it's flexible. Going to all the trouble to insulate and heat the hose so it can remain connected all the time usually ends badly for people. The water spigot does need to have heat tape on it.
Lease a 100 gallon propane tank and make sure your local service company will come to you and fill it. You may have to plumb it into your system yourself or hire someone.
Get yourself a Little Buddy propane catalytic heater. That lil dude will heat your house extremely well. Even at -10F. But it produces H2O and needs oxygen so a window always needs to be open. Plus they're not very easy to control the temp on. Sort of a back up system. Uses much less propane than your onboard furnace. And yes, you can have them indoors. They don't produce hardly any CO2.
I stayed in my RV in Fairbanks, Alaska until it was 20F during the day, and in the teens at night. But when it started dipping into the -10s during the day, I rented a cabin.