Since 1940, the only incumbent losing at this point in the cycle who would go on to win another term was Harry Truman. He, like Trump, was down around 10 points to Thomas Dewey in the early summer of 1948. But remember, Truman was not elected president before taking the 1948 election. He ascended to the office through the vice-presidency, after Franklin Roosevelt died in 1945.
In terms of elected incumbents, Jimmy Carter was the one to be down by as much as Trump is right now. Carter went on to get crushed by Ronald Reagan in 1980.
George H.W. Bush in 1992 was the other elected incumbent to lose reelection since 1940. At this point, he was ahead of Bill Clinton, though he found himself trailing in a number of polls to independent Ross Perot.
Both Bush and Carter had similar approval ratings to Trump now.