Here in the Washington it is illegal to fill a rusty propane
tank--period.  Any factory conversion like the Westy will usually not
prompt any inquiries into the condition/status of the tank, lazy, or
non-confrontational attendants don't usually go looking for trouble.

The attendants can't see very much of the Vanagon tank, so I've never
had a problem anywhere in the west with my rusty tank (no salted roads
here though), but rusty portable tanks will be refused every time, with
the admonition to just repaint it and then they will fill it.

These tank walls are pretty thick, so unless you have big, thick, flakes
of rust peeling off I wouldn't worry about it. If you do, get a new or
good used tank.
Otherwise, give it a wire brushing and paint it with a good "paint over
rust" product like Corroless from Eastwood or POR-15, then apply a cheap
spray can color coat of white or gray if you really want it to be
pretty.  With these paints there is no need to sandblast down to clean
metal, just remove loose rust and scale, their chemistry converts the
surface rust to a stable compound and seals it.  This will last you
forever as long as you have a front mud flap to protect the tank from
"road grit blasting", and you don't even need to remove it to do this,
but it is easier if you do.

Stuart