Americans are somewhat losing their regional accents, too many people move around and also due to the influence of television. My parents were from Texas, but they moved to California and consciously got rid of their accents, Texans were looked down upon at that time (late forties), but I could still pick it up from a few things my dad said, likes hep instead of help. A Texas twang is similar to southern, but not exactly. In Texas when you walk into a store the clerk will always say "how ya'all doing?", it's very cute. I am a California girl, so no accent.
If you want to hear a distinctive American accent, watch the movie Fargo. They got a North Dakota/minnesota accent down. Not as distinct as a southern accent, it's as much the words they use as the way they say it don't-ya-know.