Cblesser,
Winter camping is incredible, super pumped for you! I use my FWC mostly in the winter with A LOT of lessons learned over 150+ nights in the cold.
I can't agree hard enough with the recommendation to winterize your water system and use jugs. You can also have one dedicated gray water jug if you think you'll have a lot of excess from cleaning, brushing teeth, etc. I try and minimize that to the max though by spitting outside and using towels to clean dishes.
Running the furnace while you're away from the camper will be necessary if its 15-20 deg F if you don't winterize. IMHO its not worth the risk of damaging your water system. Drain it, blow it out, leave everything open, and enjoy it in the spring.
I sleep with the furnace off, window near my head cracked, both roof vents open (precip. dependent) and the powered fan on low drawing air out of the camper. Condensation management is the name of the game. I usually wake up to a totally dry camper with this setup.
Leave Nalgene bottle lids open overnight so they dont crack from expansion.
Pre-fill whatever you're going to make your coffee in in the morning before sleeping. That way when you wake up to a Jet Boil or pot of frozen water, you just have to fire up the stove to melt it. I'll usually then sacrifice some of that hot water to my Nalgene to thaw.
0 degree sleeping bag and a warm down blanket is sufficient for me down to about 10 deg F.
Down booties are probably the best piece of cold weather clothing I own, even with a little insulation, the floor of the FWC will be cold and can be pretty uncomfortable when you're just hanging out or standing up cooking.
Reflectix everywhere. I cover all the windows as well as the low areas where the tie-down access panels are. Helps A LOT. When in doubt, put reflectix on it.
Pee bottle. Nuff sed.
I have marine mats in a rigid plastic doormat with about a 1/2 inch high lip around the sides to drain snow and water off my boots when I get in the camper, easy to pull out and drain once my boots are dry.
Hope this helps, have fun!