Heh, this is the issue, which makes me also love this stuff. I pronounce 'awn' and 'on' in 'lawn' and 'on' the same. Grew up in Pittsburgh so that tends to do it.
Take a look at the map in the original article. The areas covered in either blue slashes or blue hatches (i.e. left and right slashes intersecting), which is most of the US, has "Don" = "Dawn". There's also the "on" line, so for those who distinguish "Don" and "Dawn", some Americans pronounce "on" as if it was "awn".
For me "tonic" has a fairly short vowel in the first syllable. Whereas "Tawnic" would have a longer vowel. There's also a different in sound. You might hear my "aw" as if it was a long "o" sound. The Australian vowel system is really nothing like the American vowel system.
My wife is from Boston too.
I love stuff like this, I've lived in Texas 13 years and have been pronouncing taco wrong until pointed out to me.
They say itmore like tocko than tacko like Brits do.