I installed a Panasonic Genius Prestige NN-SD372S Microwave Oven
in my 21 to replace the stock microwave that had a burnt out lamp (don't ask). Thanks Tom (tif-texas) for the suggestion
The advantage of the Panasonic Prestige microwave is it is an inverter type. It is a 950 watt microwave, but if you turn down the power it actually draws less. I've had it as low as 300 watts. Handy if your batteries are down when dry camping. It also does a better job of defrosting than a standard microwave b because the low power doesn't tend to burn or cook the edges of food.
You do have to trim about 3/4" off the width of the microwave cabinet, and add your own (or use the old ones) brackets to hold it in place. Be careful drilling the holes for the pop rivets - there are nasty voltages inside and parts that wouldn't like a drill bit. Same with the metal chips left after drilling. I removed the microwave metal shell to do mine.