Unfortunately Keith, it sounds like you've got the dreaded rusted tunnel
problem common to the vanagon gas tank.  This happens when dirt
accumulates between the duct hose and the top the tank.  Water makes a
home in the dirt and happily goes about it's business eating through the
metal on the tank.  You can pull it and try cleaning up the rust and
sealing it with an epoxy sealant, but usually the only long lasting fix is
to replace it with a known good used one.

I posted a primer on replacing the tanks some time ago, should be in FAQ
somewhere.  If you can't find it, let me know and I'll dig it out and send
it to you.

You could be lucky and just have a torn or dried out seal for one of the
hoses, but the only way to be certain is to drop the tank and check it
out.

Jack