After dropping skydivers the pilot entered a descent to return to the airport for another load of jumpers. He was receiving radar advisories during the drop from a tracon controller. As the aircraft descended below 4,000 feet msl the controller terminated the radar advisory service. Immediately after the controller discontinued the service he asked the pilot to check for a stuck microphone switch. The aircraft continued its descent and collided with a camping trailer and building under construction in an extreme nose low attitude. The elevator trim was found in the full nose down position. No other malfunctions were found.