You can buy a converter to do what you want, but you'll have to either add batteries or run the engine while using the microwave to supply enough power. (If the engine alternator can supply enough power - I'm not sure.)
The suggestion of a small generator, such as one of those Hondas, is a good idea. They'd be useful in emergency situations at home, too. Check the amperage, though - I'm not sure how much power the microwave draws.
1995 Airstream Classic 30' Excella 1000
2014 Ram 2500 Crew Cab with Cummins 6.7L Diesel
Sold but not forgotten: 1991 Airstream B190
Sold: 2006 F-250 6.0L Powerstroke Supercab