RE: VoltMinder http://www.voltminder.com/
I have one, currently mounted on the dash of my Westy and set up to monitor
the charge at/going to my aux battery. (As long as it's charging then the
starting battery has to be charging, the charge only gets to the aux from
the starting battery through the 1315)
I find it a well made and well thought out device and a good value for the
price.  And it can be used to look at either aux or main battery by
mouinting it and adding a small switch to select which battery you want to
look at.