The problem with 12 volt, which MrWizard has nibbled around is often voltage drop.
The "power" drops by the square of the voltage.
13.6 x 13.6 =~ 185
11.9 x 11.9 =~142
That's approximately a 23% drop in power. So now the effective wattage has dropped a heck of a lot. 80 watts x 77% =~ 62 watts.
The higher the wattage on the device the worst the results become, as the voltage drop is larger and larger.
That said, I've had good luck with a 12 volt 7 amp electric blanket. It is "old school" and doesn't have a timer on it to shut it off.
I have now moved to a mattress pad (from Aldi's) as they were being sold for just $65 USD last year. I love it--but wish I did not have to run the inverter.
Many hopes for a wonderful 2020 to all.