DURING THE NIGHT CROSS COUNTRY FLIGHT AT FL250, THE PILOT ELECTED TO LEAVE THE COCKPIT AREA AND MOVE TO THE REAR OF THE AIRCRAFT TO ATTEND TO PHYSIOLOGICAL NEEDS. ALTHOUGH OXYGEN WAS AVAILABLE IN THE COCKPIT, SUPPLEMENTAL OXYGEN WAS NOT AVAILABLE IN THE REAR. THE AIRCRAFT WAS OPERATED WITH A KNOWN PRESSURIZATION SYSTEM DEFICIENCY WHICH LIMITED THE AIRPLANE TO FLIGHT TO 17,000 FEET MSL WHILE MAINTAINING A CABIN ALTITUDE OF 10,000 FEET. AFTER 2 HOURS AND 10 MINUTES, ATC DECLARED THE FLIGHT TO BE 'NO RADIO'. AT 3 HOURS AND 57 MINUTES AFTER DEPARTURE, THE AIRPLANE WAS PLOTTED ON RADAR IN A DESCENDING LEFT TURN TO GROUND IMPACT. FUEL STARVATION RESULTED IN POWER LOSS TO THE LEFT ENGINE. PROPELLER SIGNATURES INDICATED POWER ON THE RIGHT PROPELLER AT IMPACT. THE PILOT'S BODY WAS FOUND IN THE AFT CABIN AREA ON THE AFT CABIN BULKHEAD.