The work is created exactly as they created it, and therefore truth exists insofar as the work exists. Yet this is as far as truth can be observed in a work of fiction, because the narrator/speaker is always different from the author, and therefore the text we read as relayed by this narrator/speaker is only as reliable as they themselves. This is made explicitly clear in stories told from the first person perspective; often, the foibles and fallacies of the narrator are an essential part of the story, both for the reader and the characters and plot involved. The sense of play that a lack or incompleteness of truth allows is wonderful, and something I dont think I would ever trade for having concrete beliefs about reality.
This is not to say that I believe in a total disregard for what we — or I — perceive as reality. This would be an impractical way to go through life; there is so much consensus about so many things — mathematical facts, what the sun looks and feels like, why trying to walk across a burning highway with speeding two-ton combustion engines hiding behind huge steel frames and fiberglass careening towards your spindly flesh-and-bone frame is a bad idea — that it would be pure idiocy to regard them with total skepticism.
Regarding them with the belief that there is any actual proof of their existence is equally foolish, however. Just as in literature, we are limited in the world by the perception of the narrator. In this case, the narrator is us — or at least some part of us, or parts of us, that does (or do) the processing of perceptive information and reasoning functions. and, like in a literary work, we are limited by what that narrator chooses to see and describes, and how they do it. We are unable to know whats happening just off the page or just out of our awareness. These are not things we can even be made aware of; they are simply not a part of the narrators scope. As long as there are unseen margins, we cannot make any claim about truth, and to be honest most experience is margin. The most we can hope for is to scribble a few interesting notes there as we float by..