Hot dogs, apple pie and hamburgers are usually cited. Here in Texas, we love tamales and chicken fried steak. JustaskSusan is correct.
I would like to add the US celebrates the food of our immigrants. I even think hot dogs and apple pie have German origins. I am not clear on where hamburgers came from. Pizza restaurants are found in almost every town, along with a Chinese restaurant. If you want to get technical, popcorn is a native american food. Roast beef is very popular although I have never tried Yorkshire Pudding. The foods of Thailand, Vietnam, Morocco, France, England, and Spain are just a short drive from where I live. I saw a food truck selling Korean Tacos. Maybe that is new national food.