We ran the pictured (lower left-hand corner of the pic) 1500w heater for the duration of our stay at Catalina SP (Tucson, AZ) last January; and it (along with the furnace) kept us reasonably warm. Granted, we also had four 100lb hairy heat producers to assist with the warming, but the heater ran all night without blowing a fuse or other issue. If you notice, the temp outside at the time the picture was taken was around 14* F.
The forecasted lows were only supposed to be in the upper 20s, so the extreme (for AZ) cold was unexpected. Although I wouldn't recommend exposing a Roo to those types of temps without winterizing, ours weathered it well and we experienced no damage from the cold.
Back to the original questions: Our experience is that a 1500w electric heater is not a problem to run while on shore power.