Did they find anything that they are using against you? If not, then it does not matter. However, the police have the right to do a cursory search for weapons in your wingspan so to speak (areas that you can reach) when the arrest you on a warrant.
Once you were in custody and handcuffed, the officers could seize only that which was in plain sight. An arrest warrant does not authorize the police to search the house, much less a camper outside of the house, nor does the fact that you may have had an alleged controlled substance on your person. The "wingspan" exception would not apply after you were restrained. In short, the police were obliged to seek a search warrant before doing what you described.