1.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. 2.Cover And Cook The Pork On Low For 6 To 8 Hours. 3.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. 4.Bring To A Boil Over High Heat, Then Reduce The Heat To Low. Cover And Simmer For 1 Hour, Stirring Occasionally.