Catalyzes the reduction of D-fructuronate (D-FruA) to D-mannonate (D-ManA). D-mannonate + NAD(+) = H(+) + keto-D-fructuronate + NADH kcat is 4.2 sec(-1) with D-mannonate as substrate. Carbohydrate metabolism. Belongs to the mannitol dehydrogenase family. UxuB subfamily.