>But there are two problems (at least on my camper):
>The relay is powered by turning the ignition key. So the AUX. battery
>is switched into the car's circuits during the engine start process.
>The AUX. is discharging the primary during this time but this is not
>the intention of the disconnecting relay.

I was going to jump on this point yesterday as well as the '78 has
the same system and the manual mentions this, but looking at bentley
shows the relay controlled by the D+ terminal of the alternator, 
which might in fact not reach proper voltage till after the engine is
spinning a tad bit, anyone know?  But my '78 manual says that the
batteries will be instantly in parallel discharging the good and 
arriving at a median voltage, so I guess I believe them.  Good solid 
state (diode) isolators are available from Autozone, PepBoys, Whitney
etc. in the $20 range, make a lot more sense.

John
vwbus@netbiz.net