Helge,
    I think you possibly have several problems with this van. First of which
I would increase the current carrying capability of the wire between the
alternator and the starter. This is a weak link that VW upgraded in later
years.
With all electrical consumers on high, the voltage drop across this wire was
too high. Hence insufficient charging voltage was getting to the battery.
I paralleled an eight gauge wire to the existing wire to resolve the
problem.
Ken Lewis
http://neksiwel.20m.com/