We keep a 1500 watt electric heater turned ON in our RV all the time while it's in storage in the back yard.
We set the heater to about 50 degrees and use a 50 foot extension cord from an outside house outlet to power the motorhome while we're storing it. We can also power the RV's air conditioner in the summer using this same arrangement. Every 15 amp outlet in our RV can power the electric heater at full power.
Why do some RV's not safely power 1500 watt heaters from their regular 120V 15 amp outlets??? Strange, if true.