I'm in a quandary re this, in that most meatball recipes seem to have the meatballs be made and cooked separately and then put in a tomato sauce for, say, ten minutes -- in the few recipes I just looked up. (I'm lazy re making meatballs.)
But it looks like you're interested in a slow cooked sauce, which a typical Bolognese sauce is - they can be cooked for hours at slow simmer.
And the only bolognese I make is Marcella Hazan's, since I like it. Takes a long time though fairly simple, involves cooking meat in some milk, a wine reduction (I forget in which order, which is probably important, then adding tomatoes, bla bla, and cooking that at mere simmer for at least four hours. ) Anyway, not really what you're looking for, as the meat is ground and the sauce isn't very tomatoey. But
I think a lot of other people's bolognese ragu sauces are both slow cooked and tomatoey. and perhaps some of them use meatballs.
Will look around and be back.
When you cook the meatballs separately, there is a juice left with them, and that could go in the sauce...
what I don't know is if there is a way to do it with meatballs first, transfer that to crock pot, add tomatoes et all, and then let it simmer away. And if you did, would the meatballs be sorta deadish.