In a large skillet over medium-high heat, cook the ground beef and diced onion together, breaking the meat apart as it cooks, until no pink remains. Drain off any excess grease.
Transfer the cooked beef and onion mixture to the slow cooker. Add the black beans, kidney beans, corn, diced tomatoes, and rotel tomatoes.
Sprinkle the taco seasoning and ranch seasoning mix over everything. Pour in the beef broth and stir well until all ingredients are evenly combined.
Cover and cook on low for 6 to 8 hours or on high for 3 to 4 hours. The longer cooking time on low allows all the flavors to come together beautifully.
Stir the soup well and taste for seasoning, adding salt and pepper if needed.
Ladle the taco soup into bowls and load up with your favorite toppings like shredded cheese, sour cream, tortilla strips, and fresh cilantro. Serve with warm cornbread on the side.