Assuming you are on the road, at present, the quickest way to accomplish microwave use without a generator and without a 120V source, is to use two more 12V batteries and gang all of them together with the largest wire you can acquire. AWG 1 would be a minimum with the batteries situated closely together. The inverter should be located close to the battery gang. Connectivity to the microwave would be either the inverter hard-wired into the trailer or use of a heavy duty extension cord. If an inverter is hard-wired into the load center or to the microwave circuit, a transfer switch is used.
For me, lugging out two extra batteries and an extension cord to use a microwave for lunch is just to much work when I have a propane stove that can accomplish the same process, albeit at a slightly slower speed. Currently, I have an inverter hard-wired into my rig that is used 24/7 but dedicated only to run a small 120V refrigerator and 120V entertainment gear. I have 300AH of batteries and charging is handled by 235W of solar.
If you and the family are heavy microwave users, you may wish to look at the Panasonic
inverter microwave ovens. They use less power than conventional microwave ovens.
