Since Itachi Uchiha always said complete truth, is it possible that he himself knew the truth about Tobi being Obito and not Madara?
While it is difficult to say whether Itachi later discovered Tobi was Obito, the light novels Itachi's Story make it clear that in the beginning he truly believed that Tobi was Uchiha Madara.
His reasoning, from pages 53-54:
Over the last three years, Itachi had wondered any number of times about the man's true identity. From what he could infer, Itachi had thought the man was perhaps a member of the Uchiha clan, but that changed to a certainty now.
When he thought about it like this, the possibilities naturally narrowed. The first was that a member of the Uchiha clan thought to have died in the Great War had survived, and become the man in the mask. It was a general rule that if a member of the Uchiha clan died in battle, their sharingan would be brought home by another ninja. So, the man couldn't have stolen it from a corpse. In which case, it made sense to assume that someone thought to have died was alive, and coming to the compound with his sharingan still in his possession.
The second possibility was that the man in the mask was someone who had broken away from the Uchiha. This was less likely than the first option. Because since the founding of Konoha, only one man had ever broken away from the clan and left the village.
And Itachi does end up going down the path of accepting Uchiha Madara is alive, and concluding the masked man is him, due to the overwhelming chakra he sensed, and the fact that Madara's death was never confirmed. I have also noted in the text, a part where Itachi was actually going down the right path with his thinking, one which might have led him towards Obito. Due to this, it is possible Itachi figured it out later, given his time in the Akatsuki and proximity to Toby.
Itachi clearly knew that Obito was the true leader of the Akatsuki, and he also knew of and feared Obito's (Sharingan) abilities, hence placing his own Mangekyo Sharingan on Sasuke and setting it so that Amaterasu activated when in the vicinity of Obito's Sharingan. Additionally, Itachi also had knowledge of his grudge against Konohagakure, which is why he confronted Obito/Tobi/Madara before the Uchiha clan massacre. However, Itachi never clearly states his knowledge of Obito's true identity, and also, Obito states during his first encounter with Sasuke that Itachi clearly knew more than he let on. It is never stated if Itachi knows that Tobi is Obito, but he clearly knew a lot about Obito.
Itachi definitely knew that Tobi was an Uchiha, but there is little evidence, if any, to say that he knew that Tobi was Obito. Why I think that Itachi would know that Tobi wasn't the real Madara was because the amount of chakra emanating from Tobi, when the real Madara was present on the battlefield everyone knew about his presence.
Itachi is one of the smartest people in the anime. Itachi knew about Obito from Kakashi when they were in one team in ANBU - The story of the Kakashi's Sharingan. Itachi knew about the moment when Kakashi cut Deidara's hand. All of Akatsuki knew about this situation.
And if you think more about that, from the story of Kakashi about his Sharingan, Itachi is smart enough to know about Tobi's true identity. He will understand that, if Kakashi has the same time-space as Mangekyo Sharingan, like Obito, they have the same eyes, or just Obito is Tobi. Itachi isn't foolish. Itachi was thinking about that things. That's the reason for Amaterasu in Sasuke's eye, after Itachi's death.