Preheat an oven to 450 degrees F (230 degrees C). Scrub and dry potatoes, then prick several times with a fork and place onto a baking sheet.
Bake in preheated oven until potatoes are easily pierced with a fork, 50 minutes to 1 hour. Remove, cool, and peel.
Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels; crumble.
Preheat oven to 400 degrees F (205 degrees C). Lightly grease a pizza pan.
Mix pizza crust mix, water, and oil in a large bowl with a fork until ingredients combine. When dough has pulled together, turn it out onto a lightly floured surface and knead until smooth and elastic, about 8 minutes. Let stand for 5 minutes. Shape dough into a flat circle and place on pizza pan. Build up edges slightly.
Bake pizza dough in preheated oven until crust is firm and begins to brown, 5 to 6 minutes.
Toss butter with garlic powder, Italian seasoning, and potatoes in a large bowl. Combine sour cream and ranch dressing in a small bowl; spread over crust. Top with potato mixture, bacon, onions, mozzarella cheese, and Cheddar cheese.
Bake in preheated oven until cheese is melted and lightly browned, 15 to 20 minutes. Let stand 5 minutes before cutting.