I assume that any comforter or electric pad/blanket cycles ON and OFF continuously while in use. This means that the total amps used for a night's sleep would be less than the usage calculated by merely multiplying running amps times the number of hours used.
Here's a link for some 12V warmers if you don't need a full comforter and if you want to eliminate the inefficiency of the conversion that an inverter introduces - especially for camping when not on hookups:
http://www.12volt-travel.com/12-volt-electric-blankets-c-90.html