RinconVTR.
I happen to use a 200 watt heater for my waste tanks. That's a little less than 700 btu's. The conversion number is 3.41 btu's per watt.
200 watt electric heater
The wattage varies with the input voltage to some extent, so when running on the inverter I get a little more wattage--and a few more btu's. On the generator it is even more so.
I've boondocked for a week at where the daily high was -27 c (-16 f), in blizzard conditions and used my RV at -37 C (-34 f).
Moving Sue and PhotoMike have both boondocked at -40.
