Jeff,
   I have to say that I have a different battery but
it behaves as yours does.   If I would not start the
van at least once a week it would discharge to a point
where it would not start.   I think that the
alternator does a marginally adequate job of charging
which is not all bad since it does not overcharge and
boil off the water.   What makes the condition worse
in the summer is that fridge fan is always eager to
consume a few of the limited joules of energy still
left in the battery to cool the fridge which you are
not even using.   I try to park with the fridge in the
shade.   I have found the solution is to plug in a
solar battery charger available for about 15
dollars(plus 10 dollars shipping) on ebay.   These
charger help hold the charge to just what you need to
start the van.  I plug mine in and lay it on the
dashboard.
Forget the suction cups.   I have a spare I can sell
you if you find it hard to locate on e-bay.   You have
to be a bit patient to get one for 15 but there always
for sale at
http://search.ebay.com/search/search.dll?cgiurl=http%3A%2F%2Fcgi.ebay.com%2Fws%2F&krd=1&from=R8&MfcISAPICommand=GetResult&ht=1&SortProperty=MetaEndSort&query=vw+solar
    or by typing in "vw solar" in the search prompt at
ebay.
         regards gary

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