I believe it is mostly a natural occurring event of many ABS plastics and depending on the "additives" added at time of manufacture, some yellow more than others. I don't think you have anything to worry about as my Dometic ref if the 5th I owned (2000 Thor Wanderer) was squeaky clean but yellowed. As a matter of fact, the shower enclosure/tub in the unit yellowed over time as well.
I have read that you can use a hydrogen peroxide solution with UV light to "reverse" the process with some success. The inside of a refrigerator would be quite a job.
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2014 SportsCoach 361BH - XCS Chassis, ISB 300 - 6.7 Onan QD 6000 Generator, Magnum MS 2000 Inverter
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