yeah, it's a common problem with the little vent hoses on top of the
tank .. the german rubber hoses don't last very long. only about 5-6
years or so.

it's a bitch to lower the tank and replace them, so most people just don't
fill the tank up all the way. it's not a "difficult" job to replace the
hoses, but it's a heavy tank, and a pain to drain it, lower it, and get TO
the hoses. :(

it is POSSIBLE that, if you live up where they use salt on the roads in
winter, you MIGHT have a rusted hole in the top of the tank. but my bet
is that it's a hose. try looking in the driver's wheelwell, with a
flashlight, and see if you can see any "wet" hoses (when you've just
filled up). you can also crawl under the bus (at home, afterward) and
look around at the backside of the tank ... you can see where the gas
has been running.

anyway, IF you decide to fix the hoses, fix ALL of them. cause if one is
leaking now, the others will leak soon after you fix the one. :)

and a thought: if the vent hoses are drying/cracking/leaking, can the
fuel system hoses be far behind?? if you have not already done so, i'd
recommend replacing ALL of the rubber fuel hoses on the bus ... from the
tank to the pump, from the pump to the filter, and from the filter to
the plastic gas line that goes to the engine compartment, and so forth.
be aware that there is a rubber hose, just over the starter, that connects
from the plastic gas line to the engine compartment wall ... it's a short
one, but it's been the first one to leak on two buses.

good luck.
joel