I received the following info in an email from Sure Power (Battery Separator Manufacturer) some time ago:
If it is clicking that is a good sign of several things to check for :
(1) Bad ground connection to the ground terminal.
(2) If there is a circuit breaker installed it could be tripping.
(3) Bad or too small cable and wiring sizing. Or bad connections to any part of the terminal and wiring.
(4) If a second aux. charging source is being used such as a battery charger may not be high enough voltage to keep it engaged or if the charging source being used on the main side such as the alternator may not be reaching the 13.2 volts in order for the Separator to kick in to parallel the second battery. The battery separator must see 13.2 volts in order for it to kick in. The Separator is totally dependent on the charge source and is only good as it outs out.
(5) Or shorted cell or bad battery.
First, I'd make sure the batteries are all fully charged. They should charge when you plug in the van power cord. Hopefully that will solve it but if not, work through the points noted above. Also, some Sure Power Battery Separators were recalled. Here's some info on that: http://classbforum.com/phpBB2/viewtopic.php?f=21&t=2021
The recall was for parts manufactured from 2009 to 2011. Your van is a 2008 but it's possible the part was replaced for some reason.