Catalyzes the dehydration of L-rhamnonate to 2-keto-3-deoxy-L-rhamnonate (KDR). Can also dehydrate L-lyxonate, L-mannonate and D-gulonate, although less efficiently, but not 2-keto-4-hydroxyheptane-1,7-dioate. L-rhamnonate = 2-dehydro-3-deoxy-L-rhamnonate + H2O Binds 1 Mg(2+) ion per subunit. The catalytic efficiency observed with L-rhamnonate as substrate is 70- and 16-fold higher than that observed with L-lyxonate and L-mannonate, respectively. Homooctamer; tetramer of dimers. Reaction proceeds via a syn dehydration. Belongs to the mandelate racemase/muconate lactonizing enzyme family. RhamD subfamily.