Jim,
I have not had the problem with my 2002. You could unplug the microwave unit before making the changeover to the inverter. Once the changeover is made then plug it back in and see if it comes up normally. This would tell you if the problem is the microwave "losing its mind" during the switching and weird voltage fluctuations, or if it just doesn't accept the waveform and output of the inverter for some reason.
I assume that this used to work at one point.... It could be hard to sort out what changed. One option you could try this with a "Beaver Friend" that has the same inverter and microwave. Use an extension cord from their coach to your microwave and go through the same power change routine. If you get the same symptom and the friend's coach does not with their Advantium then try the same thing with their microwave in reverse and you should be able to see where the problem originates. Have to then decide if changing something out would be worth it. Hope this helps.
later Ed