Beginning in 1990, year-round vacant mobile homes were included as part of the year-round vacant count of housing units. Year-round units are those intended for occupancy at any time of the year, even though they may not be in use the year round. In resort areas, a housing unit which is usually occupied on a year-round basis is considered a year-round unit. As indicated above, year-round units temporarily occupied by persons with usual residence elsewhere are included with year-round vacant units.
Source: US. Bureau of the Census
Release: Housing Vacancies and Homeownership
US. Bureau of the Census, Housing Inventory Estimate: Year-Round Vacant Housing Units for the United States [EYRVACUSQ176N], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/EYRVACUSQ176N/, July 6, 2015.