Melt butter in a skillet over medium heat; stir in onion and bell pepper. Cook and stir until the onion has softened and turned translucent, about 5 minutes.
Combine condensed soup, milk, and ranch dressing mix in a slow cooker; whisk until combined. Stir in processed cheese food. Mix in broccoli, celery, carrots, potatoes, and onion-bell pepper mixture.
Cover and cook on Low, stirring occasionally, until cheese is melted, 3 to 4 hours.