Involved in the biosynthesis of the dTDP-L-rhamnose which is an important component of lipopolysaccharide (LPS) (Probable). Catalyzes the reduction of dTDP-6-deoxy-L-lyxo-4-hexulose to yield dTDP-L-rhamnose (By similarity). RmlD uses NADH and NADPH nearly equally well (By similarity). dTDP-beta-L-rhamnose + NADP(+) = dTDP-4-dehydro-beta-L-rhamnose + H(+) + NADPH Binds 1 Mg(2+) ion per monomer. Carbohydrate biosynthesis; dTDP-L-rhamnose biosynthesis. Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Homodimer. Belongs to the dTDP-4-dehydrorhamnose reductase family.