The private pilot reported that, near the conclusion of a long cross-country flight, the engine suddenly lost partial power. He declared an emergency with air traffic control and was advised that the nearest airport was about 5 miles behind him. He reversed course and began heading toward the airport. Unable to reach the airport, the pilot initiated a landing to an open field. During the landing roll, the airplane collided with a fence and a detached garage. The airplane sustained substantial damage to the fuselage, empennage, and both wings. Postaccident examination of the airplane revealed blue-colored fuel streaking from the right wing fuel cap to the trailing edge of the flap. Both fuel tanks were empty. The pilot reported that he was aware of that the right wing fuel cap was leaking before the accident flight and that it had been leaking since he purchased the airplane about 6 months before the accident.