Boil potatoes in salted water till tender. Drain and mash potatoes. Add milk and salt to taste. Set aside.
Roll pizza crust out on a 14" pizza pan and prick all over with a fork. Bake at 350° for about 15 minutes or till barely browned.
Meanwhile, partially cook bacon in a medium hot skillet. Drain off all but about a tablespoon of the fat. Add onion and red pepper. Cook till bacon is crisp and veggies are tender. Drain really well.
Spread the mashed potatoes over the crust. Cover with the bacon/veggie mixture. Top with cheeses.
Bake at 375° for 15-20 minutes or till cheese is melted.