Have a Norcold 121x. On a trip for 5 months, freezer would go through it's defrost cycle properly - you could see evidence on the exterior by refrigerator. Last 4 to 6 weeks, frost built up quite a bit and defrost never seemed to engage.
Yes - it was warmer and certainly more humid in the latter weeks.
But is it possible some sensors were blocked? At least I assume those are sensors on the inside back of the freezer.
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