750 watts is what I use, it is a vornado with 375/750 switch and a tip over safety switch bed, bath beyond- $20
a 750 watt heater will draw about 6.25 amps of current.
house hold outlets and wiring ( like in your house and RV) is rated for 15 amps...so you are running at less than 50% of rated capacity of the wiring in the chinook and the house which is the source for the power.
the shorter the extension cord the better.
we use this heater on shore power at campgrounds- it is not blazing hot, but a nice quiet gentle waft of warmth which will do to keep our van warmish at temps in the 20's
covering the windows will help to keep the van warm.
mike