In order to get your heater to produce as much heat from 12V as from 120V 
would require wiring 10 of them in series.....NOT very convenient.

You could yse an inverter to convert the 12vdc to 120Vac...BUT since at 
120V the draw is about 10A,  the draw at 12V would be 100A (asuming 100 
percent efficiency in the inverter). 

I have one of the 120V cubes in my 77 campmobile and only use it when I 
am plugged in at a campsite.

Have you thought of a catalytic type heater??

Colin.