It sounds like a problem of getting the voltage from the alternator to the
battery (please electrical people don't crucify me I know voltage doesn't
flow but it is early Monday morning OK?).  I would inspect the alternator
wire to the starter and the starter wire and see if either of these has
corrosion in them or you might even be able to check them for resistance with
a meter.  They could be corroded internally or have broken strands in them
that you can't see.
I have a later model Vanagon starter cable if you need one (for when the
battery is under the passenger seat) for $60 and I am getting some alternator
harness wiring in for $12 each (should be here this week).
Also check the ground strap that connects the tranny to the chassis (located
at nose of tranny) as I believe that this one is the main ground for the
alternator and starter and could be causing your problem.

Let me know what you figure out.

Thanks,
Ken Wilford
http://www.vanagain.com
John 3:16
Phone: (856)-765-1583
Shop:   (856)-327-0027
Fax:     (856)-327-2242