Hot dogs/weiners/frankfurters are German in origin.
 
Hamburger is also German.  Yes, the “hamburger sandwich”  (the hamburger served on bread) is American, but at its core…. the ground beef …. is originally a German concept. 
 
Pizza is originally from Naples, Italy.
 
How about potato chips?  Not a “dish” per se, but a national food.   Maybe it would be the national snack.  It is a 100% American creation from Saratoga Springs, NY.   Potato chips are everywhere in the USA, eaten by almost every American, and not super common in other countries. 
 
 
Glenn