Originally Posted by Christopher Thwaites
In my 05 V10 one battery is under the hood the other is on the frame under the side door. There is no relay. Both batteries function together. Disconnect the battery under the hood and the entire electrical system is disconnected, ditto the battery on the frame. Both batteries discharge together.
I believe the dual battery set up is standard on gas vans with rear A/C which mine has.
The 2005 wiring diagram doesn't show the 2nd battery being standard with rear AC.
I found my mistake. I mistook the battery bracket that the relay is mounted to as the Aux Battery bracket, when it is the primary battery bracket it is mounted to. The 2nd battery for a gas engine is on the frame rail beneath the side door. The relay is on the under the main battery on the side of the battery bracket.
If you follow the battery cable carefully from the aux battery towards the main battery, you will probably find the relay below the main battery, on the narrow dimension of the battery.