I ran the heat in my 31-ft "A" this past weekend because my son was hosting a sleep-over. I fired up both furnaces at around 10AM Saturday morning when it was around 10 degrees F. Temps climbed throughout the day to the low-mid 30s and hung out there through Sunday afternoon. I left the furnaces running the whole time. It was very comfortable, if not too warm inside the RV. The new, 20 lb. cylinder I installed ran out early evening on Sunday (my frame-mounted tank is empty so I use a Sturgi-Stay).
So, like others said it's possible if you are prepared to spend a lot of money on propane. I wouldn't want to stay for more than a couple of days in sub-freezing temps. If my frame-mounted tank were full I wouldn't have any trouble running out, but it would start to get expensive.