At 1708:48, while cruising at 6000', a pilot of N711Y advised ATC, 'I think I'd like to turn around, head for Texarkana here, I've got a little problem.' He was provided a vector & advised of closest airports. Shortly after, he stated he would be unable to reach the airports. At 1711:49, he said there was smoke in the cockpit. While landing in a field at 1714, the aircraft hit wires & a pole then continued into trees where it was extensively damaged by impact & fire. The crew egressed thru the cockpit windows. The passengers did not escape. During flight, the crew was unable to start the cabin heater; despite repeated attempts by the captain. Smoke then entered the cabin. Fresh air vents & cockpit windows were opened, but smoke became dense. The crew had difficulty seeing. The oxygen system & hand held fire extinguishers were not used. Fasteners for the heater door were found unfastened. Examination indicated the fire originated in the aft cabin area, right hand side, at or near the floor line. The ignition and fuel sources were not determined.