"Hey, what are all of you doing in my house?"
"Hey, what're y'all doin' in my house?"
the latter sounds like something you would hear more often in today's conversation, whether it be in the south (not where it originated, btw) or in Canada, and it is still a real word. not just something a bunch of rednecks in Louisiana and/or Texas use all the time. Which they don't; stereotype, nothing more. And y'all's is only when describing something that belongs to a group of people that you are currently addressing, e.g. "Is this y'all's table, or can I move it out of the living room to bring in the sofa?"
...Or something along those lines.