Correct me if I am mistaken, but you appear to want an insight into the lives of the lower classes of the pre-modern world. I cannot think of any diaries or letters which have been preserved but something that may interest you is the Satyricon. It is a Roman prose-tale revolving around the life of three Roman's of lower class, though they are something more akin to petty thieves, con men, and general practitioners of the minor criminal arts. They constantly struggle to make ends meet, and vagabond around the empire in search of opportunity whilst fleeing from the law. The novel is comedic in tone and thus not a naturalistic portrayal, but it is enlightening, and one of the most peculiar things is how incredibly modern these three protagonists and the people high and low they meet are, every one bearing an uncanny resemblance to someone you most likely know now 2000 years after the book was first written.

Also, though the authorship is dubious dubious, many believe the author is Petronius, an aristocrat of Nero's court. So it will naturally carry the flaws which may be expected when an aristocrat tries to write of the lives of poor outlaws.