Preheat the oven to 350 degrees F (175 degrees C).

Drain pineapple juice and mandarin juice into a liquid measuring cup; stir to combine. Set fruit aside.

Place pork tenderloin into a shallow 9x13-inch baking dish. Sprinkle brown sugar, chili powder, garlic powder, and black pepper over tenderloin, then spread with pineapple preserves. Arrange onion and bell peppers on and around tenderloin. Pour at least 1/2 cup pineapple-mandarin juice into the baking dish.

Cook in the preheated oven for 25 minutes. Spoon mandarin oranges and pineapple over tenderloin and continue to cook until no longer pink in the center, 20 to 35 minutes longer. An instant-read thermometer inserted into the thickest part should read at least 145 degrees F (63 degrees C).