Just now able to get back to this. Sounds like you have good info from two very knowledgeable guys.
You can plug the cigarette plug voltmeter into those to get a simple volt reading. Don't know if your dash one will work without the van ignition turned on, they often do though. If it is in the dash, likely it is connected to start battery. Simplest way would be to check voltage for differences, but a continuity test would work as well I believe.
2008 GMC 3500 Quigley Weldtec 4x4 Savana SMB
2006 BMW HP2 Enduro-Sold