The pilot reported that he had his fuel tanks filled and that he checked them during his preflight prior to departure on a cross country flight. He experienced a loss of engine power about 3.25 hours into the flight and performed a forced landing. The airplane sustained substantial damage when the airplane impacted a "hidden gully." The pilot reported, "At the time of the engine failure, the right tank was registering slightly over 1/2 full." According to the pilot the left wing had a stain from gasoline "that had not been there prior to takeoff."