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.