This is really embarrassing to admit, but I discovered I forgot to empty my icebox last time I went camping... I obviously thought I had, but I didn't.
The smell when I opened it was very unpleasant indeed.
So I gave it a spray with the hose, poured half a packet of baking soda in it and a couple of cups of vinegar, closed the lid and left it overnight. Next day I opened it: no smell whatsoever!