During arrival, the engine began to lose power as the aircraft was on base leg for landing. The pilot was unable to restore full power and made a forced landing in an unimproved area approximately 2 miles from the airport. During the landing roll-out, the right wing tip hit a large wooden fence post before the pilot was able to stop. Approximately 1 gallon of fuel was drained from the fuel tanks during a post-crash investigation. The fuel capacity was 40 gallons, of which 0.77 gallons was unusable. After the previous flight (from Roswell, NM to Addison, TX), 27 gallons of fuel was added. The pilot stated that the fuel gauge indicated full prior to takeoff; however, he did not visually check the fuel during his preflight. Approximately 10 minutes prior to arrival at Roswell, the right tank indicated empty and the left tank indicated about 1/8 full. The total elapsed time of the flight was 3 hours and 42 minutes in addition to a 20 minute ground delay after starting the engine.