Seed and slice the green and red bell peppers. Slice the onion.
Place the beef roast in the base of the slow cooker and season with salt and pepper. Add the tomato sauce, sliced green and red bell peppers, sliced onions, pepperoncini peppers, and Ranch dressing mix. Set the slow cooker on low and cook for 8 hours.
Once cooked, shred the meat and veggies with 2 forks and combine with the sauce. Spoon the shredded beef onto the bolillo or hoagie rolls. Add slice of cheese on top and microwave to melt, if needed.
Serve Slow Cooker Italian Beef Sandwiches with a side of chips.