I see that internal rust and corrosion seems to be a Vanagon gas tank
problem sometimes.

  I encountered a similar problem on an old 356 Porsche I owned..The first
problem I found was not only was there condensation of water inside the tank
but also mold growing in there after storing the little beast for the winter
in the Pacific NW.. Every time I would dismantle the Weber carbs and clean
out the jets, within about a mile of driving, the car would start sputtering
and popping..Total mystery!  Then, I noticed some small hair-like
'filiments' were what was caught in the main jets..What the ____?  Looking
at the gas, it was a bit cloudy, then I stuck a dental mirror inside the
fuel tank through the filler cap and found the 'roof' of the fuel tank was
covered with rust and mold....

  My fix was to remove the tank and put a few shovels of washed rock, also
known as "Pea Gravel" into the tank, then strap the tank onto a paint shaker
at the boatyard where I was working.  After about 5 minutes of shaking, the
inside of that tank was shiny and bright, scoured clean by the rattling
rocks.  A few rinses and it was good to go again.  I will say, I was not too
popular with my fellow employees while that tank was a-shakin' with those
rocks inside..

  Don Hanson