You can run just about any microwave with a 2000 watt generator. Only the combo microwave / convection units need more juice.
Almost all microwaves are in the 1000-1500 watt range, and most camper models about 1000-1200. In theory you could get away with a 1500 watt generator, but few of these are made, and you need a continuous output in this range, not just peak load. Given the fact you will often have a light or two on at the same time, and will likely be providing a small amount of power to the battery charger at the same time, a 2000 watt model is the appropriate minimum.
No doubt the Honda and Yamaha models are great, but expensive as noted above. Generac makes a 2000 watt model sold through Costco for about half the price, and the Champion units mentioned are a real bargain, though a bit louder.
I have several generators currently for various uses, and the cheap 4000/3500 champion stays with my camper. It is more than I need because I never use the AC, but it runs so well and was so cheap it is hard to replace. I have been considering the Generac as a more modest upgrade than the Honda. My large Yamaha and Generac are just too big to take routinely.
Bill