You might enjoy S.M. Stirling's "Domination of the Draka" series ("Marching Through Georgia" , "Under The Yoke" and "The Stone Dogs". Reading them seperately, instead of in the combined edition, gives you a lot of the color text at the start of the chapters that the omnibus edition leaves out. John Norman's "Gor" saga gives a rationale for a Greco_Roman style slavery taking place in out time and universe, but tends to focus on breaking slavegirls abducted from Earth. Still, there are born-and-bred slavegirls there as well.