12v analogue gauge

[Daunted]

I bought a 2009 Canning second hand about 6 months ago.  I've had a few weekends away with it and the family and we are enjoying it immensely.  We did notice that on our last weekend away that when the Anderson plug on the car is disconnected, the 12v gauge shows plenty of battery but once the Anderson plug is connected it shows 0v on the gauge.  Is there any reason for this? Does this happen on everyone else's van or just mine?

The car batteries are fully charged and show good voltage, the van battery has good voltage it's just the gauge drops off when the Anderson is connected.

[Cruiser 105Tvan]

Sounds like the gauge is connected incorrectly maybe.
Job for an Autoelec.

Shouldn't drop to zero when plugged to car.
Maybe Leads from car are causing the problem?
You have checked the maxifuse, just to make sure it's ok?
Normally find the fuse on the right front just behind the outboard Jerry can position.
You do get charging current in the Anderson plug on the back of your tow vehicle?
When the Leads are plugged in the colours remain connected either side of the socket, red to red etc.
They haven't been incorrectly fitted. 
You should have another fuse in your vehicle at the battery end of the Anderson plug Leads.
That's if they've been installed with safety in mind.
The leads are a decent thickness from the battery in the Tow vehicle to the Anderson socket.
You don't have a DC to DC charger in the car that has failed?

[Daunted]

Bugger