I had the same thing happen and after taking the tank out found it was
just the vent grommet/seals that had become hard with age and leaked
with a fill-up.  You can buy new seals (3 of them I think) for a couple
of dollars each and you can reach in and change them through the front
wheel wells without taking the tank out.  The hose from the vent tanks
in the front wheel wells pushes into these grommets.  While you are at
it you may as well replace the hoses as mine had breaks in them. Better
buy a couple of spare ones as the hole they fit in on the tank is razor
sharp and you will likely ruin at least one of them.
The other possible place for a leak is the metal vent tube arrangement
that sits in a depression at the top of the tank where water can
collect.  While my tank was out I had that re-soldered and also had a
new filler neck grommet installed as I didn't want to do the job again.

Wade Yorke
84' Westy 1.9TD