My built in transfer switch defaults to the onan. If it is running, it is primary. In your example, you could start the onan, leave the honda running, run the microwave, shut down the onan, and move on. As far as the ac, start the onan, start the ac, shutdown the honda.
I have a 3kw inverter/charger on my trailer, and I still have the old 100 amp converter. I am thinking about wiring the old converter back into the 12 volt side, and put in a seporate power inlet - just for it. Then I could run whatever off the inverter, with a honda 2k running the converter - trying to charge the batteries / run the inverter. The batteries would cover any shortfall (for a limited time), and the honda / converter would be extending the time the batteries can run the inverter. When I need it, the onan is still there and can run whatever loads, and recharge the batteries fast.
