a 1200 watt microwave at 120volts should be drawing 10 amps AC .... that would be 100 amps DC 12v right?
That's essentially correct. But...
INVERTER LOSS: Assuming a good wiring plant and fairly efficient equipment, you'll come closer to the actual measured (at the battery) draw if you add about 20% (inverter loss) to the wattage when making your estimate. In your example, treat the 1200w 120vac microwave as a 1440w microwave and you'll get a better picture of what the inverter will be pulling from the battery.