Had exactly the same problem with my fuel tank.
Fuel would gush out if I filled it more than 1/2 full.
Decided to drop the tank to have a look.  The previous owner had
fibreglassed (badly) over the huge rust holes in the top of the tank and
neglected to replace the vent pipes, the fuel could slosh out the holes when
it wanted.
The main filler pipe had completely rusted through and had been 'repaired'
with a roll of rubber sheet and a couple of jubilee clips to bandage over
the hole.
It was like sitting on a potential fire-bomb.
Decided to replace the entire tank with a new one & replace / re-plumb all
the breather pipes.
The new filler pipe is made from plastic so it won't rust.
Probably cost about 200 UK pounds in all and only took a couple of hours.
At least I know now that it is safe & my expensive fuel is not evapourating
out.

Suggest you have a look sooner rather than later.
Its not a difficult job and only needs a few simple tools.

Phil, T25.

----- Original Message -----
From: Ezra Hall <ehall@TOGETHER.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Monday, March 06, 2000 9:59 PM
Subject: Fuel leaks/smell

>   Since purchase last Sept, we have had a gasoline smell problem after
fill up,
> especially when filling the tank all the way. I see the filler neck leaks
towards
> the opening, so that needs replacement or repair. However, even after
driving
> for a couple of hours, gas can be smelled when the doors are opened. I
assume
> I need to drop the tank to find the source of the leak. I would like to
minimize
> down time, are there common seals that need replacement so I can order
prior
> to dropping the tank? Hopefully we don't have rust holes on the top of the
tank...
> Other than the filler neck, there are no visible leaks from below.
> Thanks,
>  Ezra Hall
> www.together.net/~ehall
> '88 westy, Automatic