A MAN'S WINNEBAGO is not his castle.

That's the word from the Supreme Court, which decided this week that motor homes are more like automobiles than houses for purposes of the Fourth Amendment. The distinction is an important one, because the search of a house cannot be undertaken without a warrant while a motor vehicle, in some cases, can be searched when the police have probable cause but no court order.

Courts have long distinguished between privacy rights in a home or dwelling and those, for example, in a ship, motor boat, wagon or automobile. In the case of such vehicles, the Supreme Court wrote in 1925, "it is not practicable to secure a warrant (to search for contraband) because the vehicle can be quickly moved out of the locality or jurisdiction in which the warrant must be sought." More recently, the courts have given a second reason for distinguishing between cars and homes: "the expectation of privacy with respect to one's automobile is significantly less than that relating to one's home or office."

Charles Carney, whose case was decided by the Supreme Court this week, parked his Dodge Mini Motor Home in a parking lot in San Diego and used it to meet with young men to exchange marijuana for sexual favors. Was it his home or his car? Were the police justified in searching the van without first obtaining a warrant? A majority of the justices held that since the vehicle had wheels and could easily be moved on the highway, it was more like an automobile and a warrantless search was justified. But dissenters make a good case that the van was a home. It was furnished with stuffed chairs, bunk beds, tables and a refrigerator. Curtains were drawn across all the windows, including the front windshield. And it was not, in fact, moving on the highway when law enforcement authorities discovered it.

The case is a narrow one, and the ruling almost certainly would not extend to the kind of mobile home one sees in trailer parks, attached to utilities and surrounded by flower beds and playground equipment. But what about campers, houseboats and converted school buses? The courts could keep busy for decades making these life-style distinctions, but there is an easy way to limit the litigation: decide all hybrid cases in favor of the Fourth Amendment. When a vehicle is not in fact steaming out into international waters or speeding across the state line, there is no emergency. The police can get a warrant. The San Diego van, for example, was parked only blocks from a busy courthouse and even closer to a public telephone, and there was no indication that it was about to be moved. Sometimes, the best way to adapt the law to such 20th- century innovations as motor homes is to stick to such 18th-century principles as the Bill of Rights.