Muslims made up an estimated 15%-30% of Africans enslaved in the United States before the civil war. Is there a general sense of how widely the fast of Ramadan was observed among them? There were obviously practical difficulties, for example fasting while carrying out forced physical work, or maybe interference from their American masters on the religious practice.
One of the main cod-justifications for slavery was to convert the slaves to Christianity: they called slavery 'opening the gates of mercy'. It's doubtful that any Muslim slaves were allowed to remain Muslim. There were very few black Muslims in America until the nation of Islam was founded, and its members were sui generis converts.