This link below is good info on the 2nd battery wiring.  Your voltage reading from both batteries should be the same when the van is running.  The relay just connects the 2 batteries together as soon as the alternator wire (blue?) tells the relay that the system is charging.

http://www.thesamba.com/vw/forum/viewtopic.php?t=120599&view=next

--- On Sun, 12/12/10, J Stewart <fonman4277@COMCAST.NET> wrote:

> From: J Stewart <fonman4277@COMCAST.NET>
> Subject: Weekender Second Battery Question
> To: vanagon@GERRY.VANAGON.COM
> Date: Sunday, December 12, 2010, 4:40 AM
> Good Morning Folks,
> I'm trying to figure out how the second battery set up
> works in my '85 Weekender. When I bought it there was no
> battery under the drivers seat. I just installed one, and
> even though I sold off the fridge unit, I'd like to use it
> to power other things. I got some time to play around with
> it yesterday, and with the engine running, at the relay
> under the drivers seat, one lead was showing 13.94 volts (so
> that one is coming from the engine battery) and the second
> battery lead was showing 12.51 volts-shouldn't the second
> battery be charging when the engine is running? It doesn't
> seem that it is. Or does it need to drop to a certain level
> before it starts charging? TIA, Jeff
>
>
>
> Jeff Stewart
>