Anony Mous,
Actually there was a Davidic dynasty, but you are right in that it wasn't composed of the propaganda-based legends you read about in our Jewish Scripture.
There is doubt that there was a "King David" as described in Samuel and Kings. Jews regard these as legendary tales describing the government that centralized (or attempted to) the various peoples of the land which evolved into what we today call "the Jews." This dynasty of kings is responsible for making the worship of YHVH the state religion and setting up the Temple. While it is totally not impossible that there was such a person, it is doubtful the Biblical David or even a Ben David is at the foundation of this monarchy. Yet the dynasty given this name itself is not in doubt.
The Jewish take on the stories of King David is that these compose a "political apology," a Jewish form of Christianity's "right of kings." These texts were an attempt to give the dynasty "heavenly authority" for ruiling, because in all likelihood, as you mentioned, the peoples of the Fertile Crescent were not unified, neither politically nor religiously.
Like the American legends surrounding George Washington and Abraham Lincoln and the like, they are "wishful thinking," basically engendering the type of qualities the Hebrews wanted to see in their kings.
Again while this doesn't rule out a possible historical "David," my personal lineage is to those rulers who were conquered by Nebuchadnezzar and placed in the "Sephardic" region whether King David is a real individual or not. It took 20 years, but my family's been traced back to even those Jewish Christian bishops before Kyrikos who were slaughtered by the Romans for being "of the House of David" (which is why there were 16 between 40-135 C.E., as all claimed to be relatives of Jesus of Nazareth).
This also explains why many Jews, such as myself, do not accept Jesus of Nazareth as the Messiah. Since "David" is likely a legendary figure, it seems less likely that the Messiah would be a literal individual too. "The Messiah" is seen as an era of peace personified as a perfect ruler like the legendary David by many Jews for this reason.