I'm assuming your mwave works fine on shorepower and generator. Correct?
And you stated the mwave is the only thing on the inverter circuits, Correct?
You might want to try unplugging the mwave, and then starting your inverter again to see if the inverter still fails. If it does, I would then try disconnecting the output wiring at the inverter, just to be sure it's nothing to do with the wiring. If that still fails, then inverter bad.
If after unplugging the mwave and starting inverter it doesn't fail, then plug in some low wattage device like a light bulb (which would be about an amp), to see if it begins to fail under load.
DaveB, Raleigh, NC
2015 Tiffin RED 33AA, w/Honda CRV
VMSpc, Magnum BMK/ARC50