Your profile reflects the 34' Ventana, you say Magnum and PSW. Is the MS2000, MS2812, or other?
A few general comments, and opinions:
-A properly sized battery bank and inverter, can handle microwave duties, coffee pots, hair driers, residential fridges, induction top burners, etc. (As info, we replaced our 04's Modified Sine Wave OEM inverter/charger with the MS2812. We also upsized our battery bank to X's 4 L16's Lifeline AGM's for 800AH's of bank power. We have 1200W of high efficiency solar feeding them. My rule of thumb with the DW is the Microwave can be run as long as she needs it during the daytime, with strong sunshine. Cloudy days, or shady campgrounds, I ask her to limit the time to 15 minutes and under. (I'd guess she uses it on average less then 5 minutes, 85% + of the time.) This is a convection microwave, and I do ask her to only run the convection oven part with the generator running.)
-Suggest you get your batteries completely charged up to 100% SOC. Then run do a test run on the microwave, and see if you get the same results. If yes, then you have the troubleshooting steps mentioned above.
-If your batteries have been abused, even AGM's, you could have one that has gone bad. So having the batteries load tested to confirm their health might also be a good idea. (The weakest battery in the chain, is what all other batteries will strive to join
-Do you have even 15A or 20A available to you where you're parked? That would help...
-You might also check in on the Newmar forum, to see if another Ventana owner is having a like problem.
-Magnum tech support is great, and will can help you walk thru your settings if you feel you need their help!
Best of luck, and please come back and update what you find out...
And as a low number poster, delayed welcome to the gang!!!