Do a Google search for "12 volt microwave" and you will find them. They
should be more efficient than running off an inverter where you will
lose 20-30% from the conversion. A 12 vdc microwave with a 1200 watt
input will pull 100 amps from a battery. A 120 vac microwave powered
from and inverter will pull at least 120 amps for the same power. In
general, you are always better off with 12 vdc devices if your supply in
12 vdc.

Ron