We ran a microwave on a 110 amp battery, through an inverter once.
Lasted 18 minutes - was this scientific research, no it was just two fellas kipping in a works caravan who had some takeaway meals to warm up while working away from home.
400 watt inverter is not big enough.
A small microwave is 6 - 800 watts.
Minimum would be a 1000 watt inverter but I've no idea what that would do to your battery - as far as draining it.