A verbal relic from the Trump family's slumlord days, no doubt. It's real estate slang for housing units of very poor quality. The term appears in that context in
Maggie Smith's poem "Good Bones."
But there may be a specifically Long Island connection to Trump's familiarity with the term, as well. My father, who grew up in rural Iowa and Minnesota, used "shithole" to refer to
a cesspit or cesshole beneath an outhouse/latrine. Astonishingly, though, most households in Suffolk County, Long Island, New York, still use cesspools for waste drainage. Here's
an article from November 13 on the subject.