Unfortunately, this split wasn’t covered in the prior link and I have been unable to find the article I read years ago explaining the split between casserole and hot dish in Wisconsin. I did, however, find a fascinating article that discusses the east-west divide in Wisconsin. You can read it here.
What it boils down to is this. If you live in the east, you’ll use words that Cam use, like bubbler, soda, and casserole. If you live in the west, your language is much more Minnesotan, which is where you’ll say drinking fountain, pop, and hot dish.
A quick note regarding Cam’s slightly panicked question in the excerpt. Since Cam had traveled a little before joining the Marine Corps, she’d learned a few of the regional dialect differences. However, she wasn’t experienced enough to know what to expect from Sharon’s life, hence the second guess about casserole. At this point in her life, she knows that it’s easy to unintentionally cause harm, but she doesn’t always know what will offend someone. And that’s very important to her.