"When a motorhome is in a campsite set up for camping, it is a house. Same rules apply as apply to a house and that means a warrant is usually required except under certain conditions."
Well, that depends what you mean by "set up for camping." Just sitting in a campsite doesn't make it a domicile. It has to be hooked up in some way (water, elec, or sewer) to be considered a domicile and to avoid the motor vehicle exemption.
From Wikipedia:
The motor vehicle exception was first established by the United States Supreme Court in 1925, in Carroll v. United States. The motor vehicle exception allows an officer to search a vehicle without a search warrant as long as he or she has probable cause to believe that evidence or contraband is located in the vehicle. The exception is based on the idea that there is a lower expectation of privacy in motor vehicles due to the regulations under which they operate. Additionally, the ease of mobility creates an inherent exigency to prevent the removal of evidence and contraband. In Pennsylvania v. Labron the U.S. Supreme Court, stated, “If a car is readily mobile and probable cause exists to believe it contains contraband, the Fourth Amendment permits the police to search the vehicle without more.”