In my 1999 class B, I have a 2004 Tappan TM5030W microwave that measures 17.5" wide, 8.875" high, and 12" deep - .5 cu ft. The first time we went camping in the B (new to us as of October 2014), the microwave didn't work. I drove myself crazy trying to find a replacement and I could not. So, I took the faulty microwave to an appliance repair place. They could not guarantee that they could fix it. I then took it to an electronics repair place and they fixed it. I have no idea what I will do when it can no longer be fixed.
Even this one is too big for the microwave hole I have!
http://www.walmart.com/ip/Proctor-Silex-0.6-cu-ft-Microwave-Oven/44463328Some dimensions provided are inside dimensions and some are outside dimensions - I actually ordered a different one from WalMart and the inside would fit, but the outside wouldn't!