I have been mulling over what your battery sales man said about NOT 
using a deep cycle battery in your bus.The only reason I can come up
with (so far) is that those batteries will not tolerate a high current
charge rate.They are designed to provide a relatively low current output
for a long time.Conversely they require a lower, ideally about one tenth
the rated output current, regulated current/voltage charge.Your car
battery,on the other hand, is designed to provide a high current for a
short amount of time.Likewise it can take IN a lot higher amperage
charge.SO if your second battery was totally drained and you started
your engine the full output of your alternator (mine is 90 amps) would
try to enter the deep cycle battery.This would overheat the plates and
damage them over repeated cycles.
The solution ,as I see it, would be to add a current regulator from the
alternator  to the deep cycle battery. I would imagine such a device 
could be purchased from a RV or boat supply store.
Another source of energy to charge deep cycle batteries,my personal
favorite, would be SOLAR CELLS! I am surprised I haven't seen more on
this subject from all you Wesfalia owners.

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86Doublecab pickup
Ken Lewis  kdlewis@juno.com
85GL
60 356(almost finished)
61 AH Sprite (for sale)