Hi Steve,
RV's don't have much in the way of insulation in them. They tend to also be full of holes. Window area per square foot is much higher in an RV than in a home.
I'm currently drawing about 2950 watts from two 15 amp circuits.
One is running a fan heater Voltage is 105. Watts are about 1613. I have a 50 foot #12 cord, on that circuit.
One is running 1337 watts on two oil filled heaters and the converter. I have a 25 foot #12, on that circuit. I don't monitor the voltage on that circuit. The heaters in the bedroom are cycling.
Temperature outside is -9.6 c (14.7 f) Inside it is 21 c (70 f).
I will "loose the battle" in the front of the RV as I'm expecting -14 C tonight.
The bedroom, which has a sliding fabric door will stay pleasantly warm.
I'll cook tonight using my inverter--and then overnight the converter can recharge the bank for me.