Longtime White House chief usher dishes on table tennis with Carter, the benefits of Trump’s ballroom — and the time he handed Nancy Reagan a bill

He knows virtually everything about the presidents — from the toothpaste they use to the mattress they sleep on — and now Gary Walters, the White House’s longest-serving chief usher, is opening up about his decades-long career. ITK caught up with Walters, who worked at 1600 Pennsylvania Ave. from 1970 to 2007, ahead of the…