I think most of the posters here are coming close but missing one important point. Your heating source. There is an excellent reason to go away from direct-fired propane heat. Propane is a "wet" gas and burning propane in your RV creates a lot of moisture.
For a real cold weather set up you need to look at heating the RV indirectly. Espar, webasto these are all good examples, the large high end RV's use hydronic heating where a diesel or propane fired boiler circulates either water or usually glycol around to radiators throughout the camper IE (
Aquahot ) . With this kind of system you could also do a heated floor if that would work better. Cuts way down on the moisture addition to the inside of the camper. NuCamp is trying a system from ALDE that is propane fired.
If you want a real "winter camping" unit you would probably need three things:
1. Envelope without or very low thermal bridging: IE composite construction
2. All water and critical systems inside the envelope and heated
3. Indirect heating source - may be wise to have a backup as well
The high end manufacturers all do this though the products are expensive. Off the top of my head, BlissMobil, GXV, Earthroamer, Earthcruiser, Overland Explorer, or Total Composites (Victorian here on ExPo) if you wanted to build your own.
Could possibly get there cheaper and easier with a properly insulated frame of some type. I've seen a number of Bus builds that framed the interior in wood and spray foamed everything. So lots of options. Modifying a production unit of any kind only gets you so far.
For reference - I camp in snow and cold fairly often with my 2014 Outdoors RV. Well insulated for a production model but only really good down to 0F after that it gets uncomfortable and the propane burn and power associated with the furnace fan make it untenable for long stays.
I will be doing exactly as I've described above for my next build for this reason.