You might find this thread interesting, as it goes into the historicity of King Arthur.

This doesn't answer your whole question, of course. What bits of historically accurate information we can glean from the Authorian legends about late antiquity are only part of the historical value of these legends. I'd actually argue that the chief historical value of legends like those of Arthur is what they tell us about the time they were written. Authorian legends are full of depictions of courtly life that is far more appropriate to the Middle Ages than to the late antiquity of any historical Arthur. This is to say nothing of what the legends reveal about certain ideals held by the culture writing the legends; Authur is a great resource for understanding the ideals of chivalry, for example.They tell is something about how a culture viewed the ideal knight and monarch, and this is no small thing when trying to understand the culture of, say, well off folks in medieval France around the time The Death of Arthur was written.

This is all a bit outside my expertise, however, so hopefully someone else can elaborate further.