Neither had any twang that I recall. My father tended to pronounce “creek” as “krik” but that's about it. My mother always pronounced “Brea” as “Breeya” but again that's the only anomaly I remember. She did have an older sister who added “R” to words like pronouncing “Washington” as “Warshington.”
I do know that Southern California was so full of Midwestern transplants that Los Angeles was derisively called “Double Dubuque.” I always thought that the flat Midwestern accent was the basis for the standard media style.
Speaking of California, there are some words such as crayon, which are pronounced differently than elsewhere.
For example, my kids, born and raised in California, pronounce the word crayon, as “crain” as a one syllable word. My wife and I with midwest and east coast origin, say that word as “cray_yonn” with two distinct syllables.
Maybe we are saying the same thing differently. I might not be describing it right. The earliest flat speech that is spoken by regular born here Angelinos (that arent Kardashians) has no or few dipthongs in the vowels. Like the word bag has a very quickly pronounced, brief, a sound as in cat. The twang, like you say, probably did come from all the Midwesterners who migrated west, but if you hear the movie stars of the 1930s in interviews, they all have a little twang, where the word bag would be pronounced with a longer vowel, like ba-yg.
The first recorded evidence of Angelinos speaking flatly would be the classic announcer voice (ted Baxter style or gary Owens) used by broadcasters. Now we all talk like that here. (Unless coming from another country or state) (or Bakersfield! Still got some nice twang up there!)
Another example of a flattening of the American accent would be the Limbaugh brothers. Assuming they spoke the same way as kids. Rush went early into broadcasting and clearly flattened his vowels deliberately.
And Long Beach was called Iowa's seaport. Thousands of Iowa transplants used to attend the annual Iowa By the Sea picnic in Long Beach. Although it attracts much smaller crowds than it did in the mid-twentieth century, the event is now held in LA Harbor on a dock next to the battleship USS Iowa.
interesting about the r in Washington. Grew un in SW Indiana and everyone there does that. If I’m home long enough or have had too much to drink I start to do it again.