The pilot reported that he completed his last forest spray application for the day, and was landing on the trailer used for transporting the helicopter. He stated that he placed the landing skids on the trailer, and lowered the collective to the full down position. The helicopter abruptly tilted aft on the skid heals, and he applied up collective. The helicopter started to rotate to the right, and he was unable to counter the rotation with the left anti torque pedal. The pilot closed the throttle as the helicopter rotated 180 degrees to the right. The helicopter then contacted the side of the trailer, and came to a full stop. The engine was not running at this point, so he shut off the battery master switch, and disconnected the battery to avoid potential ignition of any fuel. The pilot reported that the trailer was parked on an uneven surface with the up side at the tail of the helicopter. He thought that the slope and his abrupt movement of the collective resulted in the tail rotor coming into contact with the trailer deck. The pilot also felt that the accident could have been avoided if the trailer were on level terrain, and he had waited to lower the collective until after establishing positive and complete full landing skid gear contact with the surface of the trailer. The pilot reported no mechanical failures or malfunctions with the helicopter at the time of the accident.