The non-instrument rated pilot in Piper PA-28RT, N29665, was using a call sign, N1104X, which was the registration number of a destroyed PA-32. He was cleared to climb to FL200, but as he continued, his speech became slurred. He acknowledged an ARTC instruction to check his oxygen and instruments and to descend to 15,000 ft, but he did not comply. After that, there was no response to radio calls. A C-130 aircrew made a rendezvous (at night), but detected no occupants or signs of aircraft control. N29665 was equipped with an autopilot (with altitude hold), oxygen, and an auto-gear lowering system. After airborne about 3 hrs (time to use 1 fuel tank), the plane began to descend and was observed entering a fog bank near the crash site. It crashed with the gear extended, elevation 6250 ft MSL, fuel selector to the empty left tank (right tank was full). The pilot was found alive, 6 1/2 hrs later. The oxygen tank was found empty, valve at "off" position, oxygen available for pilot use, but the oxygen masks were still packaged.