Actually- from the Town of Ulster zoning statute...

A. Residential districts:
TABLE 1
Customary Accessory Uses in
Residential Districts
Permitted accessory uses by right. (A building permit and site plan are required for certain
structures and uses per § 190-50.)

3. Parking of recreational vehicles, travel trailers, school buses, trucks of over two (2) tons' capacity, boats or boat trailers on the property other than in the required front setback. No vehicle may be used for residential occupancy, except for occasional guest parking and only if sanitary
facilities are available within the principal residence.

If this is a standard 14x70 trailer there is not much you can do, except buy the land and then evict them pending further construction. If they are living in a pulled vehicle, then the Town Code applies and they should be removed, along with the offending item.