Place roast beef into the slow cooker.
Mix together all remaining ingredients and pour over roast beef. Put the lid on and cook for 6-8 hours on Low.
Once roast beef has cooked, shred the roast beef. If it won't shred, cook it longer. Remove fat from gravy with gravy separator. Put remaining liquid into a small pot and simmer until reduced by half. Pour 1/4 - 1/2 cup of gravy over shredded meat.
Store remaining gravy in an airtight container in the refrigerator for another use.