To what extent can we use this text as a historical evidence to the slaves' lives in the United States?

Washington condemns the institution of slavery, he also makes it clear that none of his race hold any animus towards their former masters and even goes so far as to suggest that blacks benefited nearly as much from slavery as whites. He shares specific examples of slaves or former slaves taking care of their former masters and making good on debts far beyond the ordinary, implying that integrating the former slave into American society will surely yield a stronger union.

In context, Washington is reaching out to his white listeners and readers. He has left out much of the negative information and many of his own experiences, so as not to alienate the people he is attempting to persuade to help the cause.