Just a thought....
I'm sure you are sure that the thing is spotless, but I would pull everything removable (shelves and door racks) and run them through the longest, hottest cycle your dishwasher offers.
Pull everything out and air dry them, rather than allowing them to go through the drying cycle. That way if it doesn't help, it won't bake in the smell even more, and you won't run the risk of warping the plastic parts. After all, they're not designed for heat.
If it seems to help, it may give you some ideas on further cleaning of the inside of the refrigerator.
Then go at it with any of the various chemical treatments offered here.