This is a thread revival test

Another quick question to throw out to the group. My van has a rotary battery switch to control its dual battery setup. It was installed by a previous owner in the cabinet under the sink. The switch can select battery 1, battery 2, both or none. Is this a genuine VW accessory, a Bosch accessory or some other aftermarket accessory??? Anyway, I've been having problems with both of my batteries dying and wondered if it's not the cause. My question is, Does the starter always pull from battery 1 (primary battery) regardless of the switch position??? I'm under the impression that all this switch controls is where everything except the starter pulls juice from (accessories and the like). Does this switch isolate my primary from my auxiliary battery so that the aux. doesn't draw it down when it's low???? Thanks in advance!!!! Rod Woten '87 Westy "Wally" Des Moines, IA