I believe that American prime time has come out of its shell as far as showing a wide array of races and even sexual orientations. I'm not certain if they are never portrayed, but here in American, you rarely see an Arab or a Muslim in any of the prime time slots.
There are plenty of ethnicities that have never seen the light of day on prime time television in the United States. Tell me, when was the last time you saw a sitcom about a Lithuanian immigrant family? Or how about a crime drama among the Gullah people of Georgia? Those are just two examples.
There are many ethnicities that are not very well known. There are likely ethnicities that are not shown on prime time television not because they don't want to but because they are not well known and there are so few of them. There are certain tribes in Hawaii or smaller islands that come to mind.
I'm not about to go out and survey all that is on prime-time TV, but I would venture to guess that most major ethnicity's are accounted for, although they may be in very small roles depending on the ethnicity's presence within the United States. I'm sure if you studied long enough you would find that they are not all represented.
To be honest, I really do not watch a whole lot television, whether late night or prime-time; however, I have noticed whenever I do watch prime-time that there is a little bit of every ethnicity. No one is treated differently and I am sure there is something for everyone on TV.