Blend chili powder, cumin, salt, and red pepper. Rub meat all over with spice mixture. Place onion and garlic in bottom of crock pot, top with meat.
Spoon 1/2 cup salsa over meat, along with the rotel, cover and cook on low for 8 hrs. (6 on high). Remove meat from crock pot, place on plate and use 2 forks to shred meat. Skim off and discard fat from cooking liquid, return meat to juices in crock pot and mix well. Adjust seasonings to your taste.
Place shredded meat on warm tortillas, top with cheese, tomato. Roll up to enclose filling. Serve with remaining salsa.