Hmmm. This is not a brand-new trailer, is it? If so, I would suspect foam from the ceiling insulation installation had gotten into the a/c system.
But if it is not a new trailer, then it is possible you have (or had) mice nesting in the a/c equipment. They sometimes chew up exposed pieces of foam and use it for nesting material, since it is such a great insulator.
One year, we were camped in a meadow and left a closed cell foam mat (about a half inch thick, two feet wide, and three feet long) on a lawn chair overnight. By morning, the mice had turned it into swiss cheese.
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