The slaughterhouse is a large building with sheet metal roof and walls on the far left of the map in Lakeview Cabin IV. It has no windows and one entrance door. Above the door in big fancy letters is: Lakeview Slaughterhouse and underneath that is a logo of a pigs face. A secret sliding door in the last room can lead to the Mansion if it's generated to be there but can only be unlocked from the Mansion, the sliding doors are unlocked by either a switch or using two players, one to unlock and one to go through the door.
Sometimes going the wrong way in the Slaughter house will drop the playing in a random Mansion room with no door the get back to the slaughterhouse. If this happens just try again and go a different way.
The player has to visit the slaughterhouse and get the gas canister from Piggy's room in order to finish Lakeview Cabin IV (unless they get lucky and the gift room in the mansion contains gasoline). Taking the gas will wake up Piggy, who will chase you down the slaughterhouse. He ignores traps and always chooses the correct door, but doesn't open doors, sawing them down instead, giving you a waning he is about to enter the room.