We also had a 3124k and after 2 blowouts changed to Load range D, Goodyear tires and no more trouble.  We kept the at the full 65 pounds and checked it regularly. On a tandem axle, when one tires blows, the other tire is severely overloaded.  Other forums have had posts that reccomend replacing both.  I have a friend that only replaced one tire and later that trip it's mate blew out.