This is a strange sentence. "Working out of trunks" is NOT an English idiom or expression. I googled the context (it's a passage from "The Ultimate Book of Top Ten Lists"), and it still seemed strange to me as a native speaker. In order to understand, I had to look up "Hetty Green" on Wikipedia, which explains in more detail as follows:
"Green conducted much of her business at the offices of the Seaboard National Bank in New York, surrounded by trunks and suitcases full of her papers; she did not want to pay rent for her own office."
What this means is that Hetty Green would take all of her business paperwork, literally stick them in suitcases, and sit in the lobby of a local bank in order to do all her business. In this manner, she avoided having to pay rent for an office. This is extremely surprising, given that Hetty Green was one of the richest people in the world at the time.
Working out of trunks does not mean she worked very hard.