I've always done just a spring cleaning w/ warm water and baking soda and that's it for the whole year. When not in use, we leave the doors open. However, OP said that a box of baking soda didn't touch the odor so the wiping w/ the solution I mentioned may not work. Like others have said, check under, behind the fridge for anything that may have either got kicked under it or died behind it. Plus would agree too to perhaps use something stronger to clean inside (bleach), then wipe down w/ baking soda.
Good luck and let us know how you make out.