The 2000W Yamaha is a prime example of misleading advertising.... It'll only put out 1600W of power, continuously. If your microwave is drawing less than that (check it's manual - they typically draw about 1.5x their output wattage) try turning off the 12v converter at the panel to limit where other wattage is being used.
BTW - I have the "2400W" Yamaha, so that I can get 2000W of continuous power.....
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2014 RAM Laramie 2500 w/6.7L Cummins Diesel and Automated Safety Hitch
2016 377RLBH Jayco North Point
300W of Suntech Solar w/Rogue MPPT Controller & Xantrex 2000W PSW Inverter