I'm boondocking with a 15 amp connection at the moment. I'm running a 860 watt fan based heater (2951 btu's) so as to not blow the breaker.
Temperature outside is -5 C (23 F). Wind is 18 kph (12 mph), and windchill (yes I know it is just for people) is -8 c (18F)
The RV is a Kustom Koach 28'5" "made in Canada" unit that has a winter package (dual pane windows) but an unheated basement (so far as I can tell).
Run time so far with a 35,000 btu propane furnace is about 10 minutes per hour.
The inside of the RV is "bouncing" between 18.8 and 20.1 C