What I do is this: I use a foam sponge, and buy them at the Dollar Store - $1 for 8. While the interior is still warm, I spray an all purpose cleaner inside. Wait a minute and then use the spone go wipe it all up.
The microwaves that annoy me in my business are the over-the-stove ones. I am short, 5'3", and often cannot reach the back or top without a step stool. And when you clean for profit, that means extra time (time you probably willl not have!) Just finding such a stool may take a while unless the owner is present.