I mean sure it is possible he never existed, but Mohamed was very political and one would think his interactions with the greater political world would have left a verifiable impact. His encounters with Ethiopian kings, letters to political leaders, participation with the Islamic empire in general.
But of course I think his legacy was altered and used for political purposes by others.
Only he knows what was in his head. But I think if you look at recent political history, you're sure to find similar questions.
What was going on in Hitler's head?
What was going on in Mao's head?
What was going on in Osama Bin Laden's head?
What was going on in Sadaam Hussein's head?
Does Obama really mean everything he says in his speeches.
Heck, there was a even a case in the news recently about a young woman lying that she had cancer to get money from people.
Who knows what is going on with these people. That's why we're not there doing such things

Was he a very smart manipulative politician?
Was he deluded/mentally ill?
Was he a seeker who just got really confused?