I was just reading the guidlines for tank inspection... Surface rust is not an indicator of condemnation, simple cleaning and painting is enough
There are rules now days for propane tanks, a new tank must be inspected at 12 years of age
And every five years after that, This inspection (Recertification actually) is done by professional, trained, certifiers
And a leaking valve would necessitate repair or replacement
Note; The book I"m reading (Woodall's RV Owner's Handbook) suggests replacement is normally less expensive than repair
It also talks about how to keep the tank in good condition