Catalyzes the hydrolysis of alpha-D-ribose 1-methylphosphonate triphosphate (RPnTP) to form alpha-D-ribose 1-methylphosphonate phosphate (PRPn) and diphosphate. alpha-D-ribose 1-methylphosphonate 5-triphosphate + H2O = alpha-D-ribose 1-methylphosphonate 5-phosphate + diphosphate + H(+) The sequence shown is that of strain K12. Belongs to the metallo-dependent hydrolases superfamily.