Except the level of egomaniac behavior he has displayed IS evidence of mental illness. As for his sanity, the only time "legally sane" becomes a real issue is if someone is a defendant in a criminal or civil case and there is a question as to their ability to understand the proceedings and aid in their defense. By that measure, sure, he's sane. By any measure we should use to judge the FVCKING PRESIDENT OF THE UNITED STATES, he is clearly not competent to occupy the White House. His grasp on reality is far too flexible, his ability to understand complex international issues of trade and diplomacy is non-existent, he is a pathological liar (another clear indication of serious mental illness) and even if none of that was true, his actions and solutions will only serve to drive the country deeper and deeper into debt with none of the benefits he seems to believe will come from building a wall, or banning brown people from entering the nation, and running the military like it is his own little inconsequential game of Risk. Oh wait, what am I saying, he doesn't have the intellectual capacity to play a game as complicated as Risk.
