Do we know if John the apostle(or any of the apostles for that matter) was a real person or not?
No. But based on the highly legendary nature of the character called John, it would be reasonable to assume that some person did something which triggered the legend. For example, John is associated with a foreign word "baptism" which, had it been a Jewish rite, would have had a Hebrew or Aramaic name. So right off the bat we have to question why the Gospel of John begins with him baptizing in the wilderness, since no one would have understood what that means. If they had a good knowledge of Greek they might understand that it means he was bathing, but they would have no reason to associate this with a Jewish rite. The discovery of the ruins of Qumran, near where the Dead Sea Scrolls were found, revealed underground bath houses, which were the consequence of their livelihood, which was quarrying rock. The Qumranis were a sect of disaffected Jews who left Jerusalem and went into "the wilderness" to escape the bickering between sects like the Sadducees and the Pharisees of Jerusalem, where they could escape these material distractions and live a contemplative life. These people were the Essenes. It stands to reason then, that the story of a man baptizing in the wilderness is possibly a legend that stems from a metaphorical reference to the Essenes at large, or else there might have been an Essene named John who for some reason became a central figure in the second version of the Jesus story, so different from the Synoptic versions, which we call the Gospel of John. But of course the legendary mangling of facts and events leaves this completely uncertain.
I personally believe that the use of
logos in the opening of John, commonly translated "In the beginning was the word and the word was with God and the word was God" is a concept borrowed from Greek Stoicism. That is, I believe this translation is incorrect. I would translate "in antiquity logos pertained to theos; logos and theos were identical" which takes us into Stoic beliefs. It also appears that the Essene philosophy had much in common with Stoicism. That leaves it to decide if there were a branch of Hellenized Jews who were so steeped in Stoicism that their own legend about Jesus took on its own separate oral tradition.
Is there any proof that we know is credible?
The main proof that the author of Book of Revelation is not the same as the author of the Gospel of John is found in the exegetical analysis, which concludes that they are probably not the same authors. The earliest documentation of this was by the Alexandrian bishop Dionysius, sometime before his death in 165 CE.
and if he wasn't real, do we know who really wrote the book of revelation?
No.
This may have been a book that escaped the stage of oral tradition and was directly committed to text. Because of the highly hallucinatory nature of the visions described, it is reasonable to assume that a person with brain damage authored the bizarre story. It has been suggested that the author may have been epileptic. In the last decade or so, there has been some neurological evidence to correlate hallucinations and "the religious experience", as a syndrome of some seizures, with patients likely to describe bizarre visions.