Place grill bag, window side up, on hot grill and cook until potatoes are warmed through, 15-18 minutes. Do not close grill; heat from a closed grill will cause grill bag window to melt.
Place steaks on hot grill and cook on one side until browned, 5-7 minutes.
Flip steaks and top with butter. Once butter has melted, top with tomato sauce. Cook until steaks reach a minimum internal temperature of 145 degrees, 5-7 minutes.
If using NY strip steak, cook on one side, 9-11 minutes. Then flip, follow same instructions as sirloin steaks, cooking until NY strip reaches a minimum internal temperature of 145 degrees, 9-11 minutes.