That would be a strange phenomena how the Bible could be all fiction, written over hundreds of years, by different fictional authors adding to the story. I am having a hard time wrapping my brain around that one.

Let's say the New Testament was written over 50 years by one person. Maybe this person was so versed in the Old Testament stories, he fit the New Testament together with it really well, even matching many contemporary persons and archaeological reference points.
How do you disprove this allegation?

Since so many corroborating independent sources of apostles knowing apostles, disciples working with disciples, across different periods of time, and there is no evidence for novel theory, then this theory falls on its face. The burden is on the skeptic to show fictional theory could be true.

Ancient critics tried to refute the resurrection, indicating it was historical.

The award winning The Case for the Resurrection of Jesus by Gary R. Habermas says it best I think. "If we look at the New Testament material on Jesus' resurrection, there are indicators that the accounts were meant to be understood as historical rather than mythical [or fictional]. Consider the two sermon summaries of Peter and Paul, recorded in Acts, that contrast King David's buried body with Jesus' resurrected body" (Acts 2.22-32; 13.34-37) (88-89).

They claimed that Jesus' body did not decay in the grave as David's did, but rather was raised by God. It is difficult to imagine how Peter and Paul could have been any clearer if they meant to communicate a literal, physical resurrection. If a mythical [or fictional] genre was being employed, Peter and Paul could have easily said, "David died, was buried, his body decayed, but his spirit ascended to be with God. Jesus likewise...."

On page 294-295, we read, "Also note that Peter's sermon as portrayed in Acts 10:40-41, he claimed that he and others "ate and drank with Him after He rose from the dead." Luke seems to be intending to record historical events when in Luke 1.1-3, he writes,
"Inasmuch as many have undertaken to compile an account of the things accomplished among us, just as they were handed down to us by those who from the beginning were eyewitnesses and servants of the world, it seemed fitting for me as well, having investigated everything carefully from the beginning, to write it out for you in consecutive order, most excellent Theophilus; so that you may know the exact truth about the things you have taught."
New Testament critical scholar Bart Ehrman comments,
"There may indeed be fictional elements in the account, as we will see; but judging form the preface to volume one [i.e., Luke's gospel] from the subject matter of the narrative (the spread of the Christian church), and from the main characters themselves (who are, after all, historical persons), we can be more plausibly conclude that Luke meant to write a history of early Christianity, not a novel. Moreover, all of the ancient Christian authors who refer to the book appear to have understood it in this way." (Bart D. Erhman, The New Testament: A Historical Introduction to the Early Christian Writings, 2nd ed. [New York: Oxford University Press, 2000], 124)
See also A. N. Sherwin-White, Roman Society and Roman Law in the New Testament (Oxford: Clarendon, 1963), 188-89. "The work of Luke cannot be evaluated properly if we group it with inferior contemporary literature that treats of heroes, thaumaturges and other popular characters. It is genuine history" (G. Kittel, G. W. Bromiley, and G. Friedrich, eds., Theological Dictionary of the New Testament, 10 vols. [Grand Rapids: Eerdmans, 1981], 3:395).

Some scholars have noted that the appearance language in the New Testament is the language of sight (Luke 24.34, Acts 10.40-41, 13.30-31, 1 Cor. 15.5-8). The writers did not use metaphorical language, so they at least thought God had acted literally upon them in the appearances of the risen Jesus.

Although 2 Peter cannot be part of the "minimal facts" argument because many scholars question its authorship by Peter, it still provides early testimony that at least some Christians within one hundred years after Jesus were interpreting events such as Jesus' transfiguration and resurrection as historical events.

In summary I would say, if the intent was to be historical and not mythical or fictional, I am not sure how they could be any more clear. And if they wanted to be mythical or fictional, I can't find any evidence for this proposition from the historical record. Praise the Lord!

On page 295...
We can also note with New Testament scholar Craig Bloomberg, "A careful reading of the patristic evidence suggests that indeed the vast majority of early Christians did believe that the type of information the Gospel writers communicated was historical fact, even as they recognized the more superficial parallels with the mythology of other worlds."