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If the unit is newer You could be getting odor from the foam insulation.
If it is 2 years old(at least), and has been used regularly, the only way an odor can get in a refrigerator is a spill or a spoiled piece of food I would suggest turning the unit off and taking all the shelves and drawers out and do a very good cleaning, after it dries turn it back on and let it run for 24 hours and check it again.
If You need more information feel free to ask.
This unit could very well have a leak, the fact that You said it was in a fifth wheel.
Units in motor homes and the like can have ammonia as the refrigerant, this can be VERY dangerous, even fatal when inhaled.
You should have a trained expert take a look at it.