Maryland Code, Transportation § 11-134

(1) A trailer or semitrailer that is designed, constructed, and equipped as a permanent or temporary living or sleeping place and for use as a conveyance on highways, but that does not qualify as a camping trailer or a travel trailer as these terms are defined in this subtitle; or

(2) A trailer or a semitrailer that has a chassis and exterior shell designed and constructed as described in item (1) of this section, but that is used instead, either permanently or temporarily, for the advertising, sale, display, or promotion of merchandise or services, as an office, or for any other similar purpose, except the transportation of property for hire or the transportation of property for distribution by a private carrier.