Of course not every single American eats the same. But, probably the best reflection of the "average" American diet is via "mainstream" restaurants (i.e. chains, but not fast food).
Look at places like Chili's, Friday's, Ruby Tuesday, and (gasp) Cheesecake Factory. Their portions are out of control and they are laden with fat and sodium. It is not unusual for a single meal at those places to nearly meet or exceed one's allowance of, calories, etc., for an entire day. And some meals (ex. Just about every entree at Cheesecake) are cartoonishly huge.
It's ridiculous. But, I think that defines the "average" American diet in the sense that these guys are all targeting the mass market. So, what they serve is the product of much research into what people want, and then is constantly being tweaked based on observation.
Let me get this straight: you are looking for healthy, sensible meals at a place called THE CHEESECAKE FACTORY. Are you serious? Read that again and think about it: the name of the place is The Cheesecake Factory. You're trying to say that this restaurant is representative of a person's overall diet in the U.S.? Completely ignoring any other options that offer healthy food? Completely ignoring cooking at home? Completely ignoring the plethora of farmer's markets, grocery stores with amazing fresh produce and lean meats, the growing popularity of the Whole Foods and similar stores throughout the country?
Oh, I see, you don't like to think about these things, because it goes against the "AMERICA IS HORRIBLE" narrative that is so popular on the internet.