AT the height of her Dallas fame as the bed-hopping Lucy Ewing, J.R.'s sexpot niece, Charlene Tilton made $50,000 an episode, appeared on 500 magazine covers and drew 65 million viewers for her 1981 TV wedding. But by the time Dallas, America's top-rated show in the early '80s, limped off into the sunset in 1991, Tilton, who had left the cast for good the year before, was just a South-fork memory. Since then, she has done commercials for the Abdomenizer, a fitness product, and been in a few straight-to-video movies. The money she made from Dallas is long gone, thanks to poor investments. Yet Tilton, 34, is upbeat. The designer of a successful swim wear line, she just returned from a tour of Eastern Europe, where Dallas, practically a primer on the dark side of capitalism, is a hit. She lives in the San Fernando Valley with 12-year-old Cherish, her daughter from her first marriage to country singer Johnny Lee. Her second marriage, to Scottish musician Domenick Allen, ended in 1992, and she is engaged to contractor Robert McGrane, 27. As for her career ups and downs, Tilton insists, "I've never been out of the limelight." But she adds, "None of that means a damn thing. Cherish is my No. 1 priority."