Preheat oven to 350 degrees F (175 degrees C). Wrap potatoes in aluminum foil.
Bake in the preheated oven until potatoes are tender, 45 minutes to 1 hour. Cool potatoes until easily handled and cut into cubes.
Place the bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon slices on paper towels and crumble.
Combine milk, cream, sour cream, processed cheese, and Cheddar cheese in a large pot over medium heat; cook, stirring frequently, until cheeses are melted, about 10 minutes. Add potatoes and bacon to milk mixture and simmer over medium heat, stirring frequently, until potatoes begin to dissolve into soup, about 20 minutes; season with onion salt and black pepper.