Spray a skillet with cooking spray and place it over medium-high heat.
Add the ground beef and break it up with a wooden spoon, stirring for about 1 minute. Stir in the onion and continue to cook, stirring often, until the ground beef is brown all over and the onion is soft, about 4 to 5 minutes.
Drain the excess fat.
Transfer the browned beef and onions to the slow cooker (or a soup pot).
Add the tomatoes, beans, corn, water, taco seasoning, and ranch dressing mix.
Cover and cook on LOW for 6 to 8 hours (or simmer over low heat on the stove for 30 minutes to 1 hour.)