Place the pork roast into an oven roasting bag set inside a slow cooker. Sprinkle the chili powder over the roast and arrange onions on top of the roast. Loosely close the top of the bag with a nylon tie. Use scissors to cut 3 vents, 1-inch long in the top of the bag. Pour the water into the bottom of the slow cooker, around the bag, so that it is at least 1 inch deep.
Cover and cook the pork on Low for 6 to 8 hours.
Remove the pork and onions from the bag and place in a large Dutch oven; reserve 3/4 cup of liquid from the bag. Shred the pork by pulling it apart using two forks. Stir the salsa, tomatoes, and cooking liquid in with the shredded pork.
Bring to a boil over high heat, then reduce the heat to low. Cover and simmer for 1 hour, stirring occasionally.