I (we) would often use TC's for snowmobiling in -30F temps.
I did the following and found things to work well as long as you're not attempting to use it with water.
I stayed in CG's with heated bathrooms/showers. I added a Curve electric flush (AA batteries) porta potty in the camper to use during the night.
Look for a non slide unit and make storm windows from 1" DOW pink board.
1. Cut 2 pieces of 1" DOW the same size of the window.
2. cut a square of any size you wish in both piece, so you can see out.
3. Place clear shrink wrap covering the first piece of DOW board then sandwich the second piece together with the first. This gives you 2" of thickness with a clear insulated square to see out of.
I just used pressure to hold it in place over the emergency window, wedged the DOW board between the handles. That way you can easily pop it out if need be. Use Velcro patches on the others to hold them in place.
Doing this worked well and shaved a lot of money off buying real custom storm windows. I experienced no condensation problems.
I cracked a roof vent 1" and that seemed to do the trick.
What I found doing this for 8 yrs or so for 20-30 days at a time :
At -20 , -30F you need the furnace and possibly an electric cube heater as a supplement for those cold spots that vary in camper to camper.
My Arctic Fox slide model with no dual pane windows used a lot of propane ( a 30# tank in 4 days) I added home made storm windows and that helped a lot but I don't remember exactly how much less propane was used. That unit had a 20,000 btu ducted furnace which struggled to maintain 65-68F. I had to close the slide at night to keep it warmer, which created another issue.
I had to climb onto the roof to clear snow from the top of the slide before I could close it. (slide topper awnings make it worse, do not use one in snow country, snow/ice gets trapped under them making it too difficult to remove)
I used that TC for 4 yrs then switched to a non slide Lance with storm windows with a non ducted 18,000 furnace and that used less propane. A 30# tank lasted 8-9 days maintaining 75F-78F. I used the DOW home made storm window over the emergency window (they don't make storms for emergency windows) An electric heater would heat that camper alone in temps around 25F plus.
At those -30F temps, its a good idea to insulate the propane regulator changeover valve so it does not freeze (gas flow will stop, carry a hair dryer :)) just wrap it with some insulation , you'll be fine.
You can ask your local gas company to set you up with 100# tanks or larger. It ends up being cheaper for bulk pricing, most of the other seasonal winter guys did it that way.
I placed a H.D. insulated curtain over the entrance door which really helped. Those doors are a major cold air leak source.
I installed 2x2 soft foam tiles on the floor to insulate the floor and that helped as well.
You can also add those interlocking 2x2 foam pads under the bed if you find it transferring cold. Use them anywhere you feel you need them, like under the dinette cushions etc.
Good luck, its doable :C
