I always thought the pronunciation was "All y'alls" :-)
Of course you are correct. (Hey - I’m a yankee!) Although growing up in Minnesota and knowing enough Norwegian I was mistaken for being a Norwegian a couple of times when I visited there, accent and all.
One time on a hike we came across an older guy up in the mountains - it was clear that he lived up there. The Norwegian guy I was with was having a conversation with him, but I could tell he was having a difficult time, and I couldn’t understand much. I asked him what the old guy was saying.
“Beats me - I could only pick up a few of the words. Lots of these mountain valleys have dialects that are so different from anywhere else.”