Sentence examples for you all know from inspiring English sources

"you all know" is a correct and usable phrase in written English.
It is commonly used in spoken English as well. You can use "you all know" to address a group of people and indicate that what you are about to say is common knowledge or understood by all of them. Example: "You all know that our company has been experiencing financial difficulties. However, I am confident that with our collective efforts, we can turn things around and come out stronger."