Generally speaking, someone living in a trailer on private property isn't a crime. It may however violate local city ordinances, land use or zoning rules or may create other issues like whether the living situation poses a fire or sanitation hazard. People do live in trailers and mobile vehicles and manufactured homes. For the most part they won't be disturbed unless someone reports it to a county official who checks and finds the living situation constitutes some type of violation. There may be other rules involved if the trailer is being rented as a dwelling.
Why are you asking this question? If you are the one that wants to know about this for some reason, you can call both the city or county where the property is located and find out what the rules are. But if there is a violation the result is likely to be that the person is asked to stop living there. There could be a fine. But absent a specific crime being committed or refusal to cooperate with public officials a criminal charge is unlikely.
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