According to the pilot, following his landing he taxied toward a fixed based operator during night, visual meteorological conditions. As he entered a general aviation parking ramp, the right main landing gear struck an unlit and unmarked helicopter dolly. The right landing gear collapsed, the right wing deformed during the collision with pavement, and a fuel leak resulted. The pilot reported that he did not see the dolly prior to the collision, and that there were no mechanical malfunctions or failures of the airplane. The pilot and the Federal Aviation Administration inspector who responded to the accident reported that the helicopter dolly was unlit and unmarked.