There are 2 ways to find out if the valve is replaceable, first, call Manchester Tank co.
with your tanks S# in-hand, second, call a local propane dealer, tell them you have an ASME RV tank with a faulty valve and ask it they will replace the valve.
I suspect the reason those RV dealers said the tank must be replaced is because they do not have an LP gas certified technician to work on ASME tanks, replacing the entire tank is different that taking it apart and replacing a component of the tank.
2000 Winnebago Ultimate Freedom USQ40JD
, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert theConstitution. "Abraham Lincoln"