According to American National Biography (Oxford University Press, 2004), Sanders (1890-1980) came from a poor farming family and his mother had to work in a factory. He never owned a plantation. His jobs ranged from railroad work to law to running a Shell station, along with other scattered gigs. I haven't yet found any information one way or the other on any possible Klan connection.
As far as "dirt" goes, these quotes from the same source might be useful:
"In 1906 he lied about his age and joined the army. He was posted to Cuba where, his true age still undiscovered, he was honorably discharged after four months service."
"Shortly after the birth of their second child Sanders began correspondence courses in law. In 1915 the family moved to Little Rock so he could practice in the Justice of the Peace Courts, where admission to the bar was not required. His legal career ended when a courtroom fistfight with a client over a fee ruined his practice."
"Sanders began to develop his "Colonel Sanders" persona in 1949 after he received his second honorary Kentucky Colonel's Commission. Exactly why he did so is unknown, but it was a role he clearly relished. The Colonel's public image of gracious hospitality and down-home charm clashed somewhat with his personal reputation for fierce independence and quick and sometimes violent temper. In the early 1930s, for example, a disagreement with a business competitor escalated into a shootout among several people that left one person dead, and throughout much of his career his explosively profane outbursts kept most of the help in his various ventures on edge."