Rinse steaks with cold water and pat dry with paper towels. Season both sides of steaks with 1 packet of fajita seasoning. Slice into strips; set aside. Add vegetables to a 5-quart slow cooker. Toss with remaining packet of fajita seasoning. Add steak strips on top of vegetables. Pour in beer. Cover and cook on low setting for 8 to 10 hours.
Serve hot with peppers, warm tortillas, refried beans, and shredded cheese, if desired.