Hi,
RV plugs are never 220 volt.
I have three shore power cords. The OEM 30, a 20 and a 15 amp. That lets me heat 100% electrically.
That said, the "stab" connectors for the outlets in RV's do not do well with heavy loads. I've replaced all but one of mine with screw terminal type.
With the oem wall plugs it may be better to not exceed 750 watts on any circuit. However there may be several circuits on different breakers so the potential is there to do quite a bit of auxiliary heating. My favorite heaters are electrical carpets. My next favorite are 1/2 size oil filled units (about 440 watts each).