I've got a 2015 U3 Cadiz which will have the same electrics as yours.
All the 12v wiring on a Unicorn 3 van is connected by male and female connectors on the printed circuit board (PCB) in the consumer unit. The PCB has 6 relays on it and 14 fuses for all the electrical items in the caravan.
I have reversed engineered how the electrics work so I am confident with the following steps.
If any of the following steps are not clear I will try to explain in more detail.
You will need a multi meter to check for volts and continuity.
Also carry out a visual check of the consumer unit to see if there are any signs of burning around the 20amp
Van battery fuse . These have been known to over heat due to a bad connection.
Carrying out the following steps should identify either a problem with the supply from the battery to the consumer unit PCB (printed circuit board) (step 1 to 4), a faulty relay on the PCB or a problem with the 12v Power switch .
Step 1 - unplug the 240v EHU from the caravan.
As long as you have Volts at the Battery its safer to have the 240v disconnected from the van while
Step 2 - Disconnect the 13pin plug from the car(If connected)
Step 3 - Switch the fridge to the mains setting. This connects the fridge interior light to the 12v
The 12v fridge supply is always live and not switched from the control panel or the habitation relays.
Step 4 - Will prove if there is 12v DC to the PCB in the consumer unit.
Open the fridge door.
If the blue light comes on you have just proved that there is 12v from the battery to the PCB and the battery 30amp fuse and the 20amp Van battery fuse are OK.
If the fridge light doesn't come on, the problem is between the battery and PCB.
Check the van battery fuse. Replace fuse if blown and repeat Step 4.
Check battery volts.
Check all connections at 30amp fuse holder.
Check for signs of burning around the 20amp van battery fuse.
Step 5 - Switch the 12v Power switch on .
Step 6 - Push the Battery volts test switch "V". If you see a voltage on the display carry on to Step 7
else go to Step 8
Step 7 - Switch on the awning light.
If it comes on the caravan 12v is OK
If it doesn't go to Step 9.
Step 8 - Carry out Step 7. if the awning light comes on the battery volts display module has failed and should be replaced.
Step 9 - With the 240v EHU unplugged and the 20amp van battery fuse removed, remove the bezel around the
3 control panel units, Remove the battery volts display module and power switch module. leave them hanging on the side of the panel.
Check the continuity of the power switch across the red and orange wire at the back of the switch.
replace if switch contacts do not close when operated.
If switch is ok, leave it in the on position.
Replace 20amp van battery fuse and with a multi-meter test for 12volts on the red and white wires on the connector attached to the battery volts display module.
If there is zero volts across the red and white wires then one of the habitation relays on the PCB has failed and a new Circuit Board required.
If there is 12 volts across the red and white and the awning light does not come on then the relay D operated from the power switch has failed. Again this will require a new circuit board.
The Control Panel Power Switch, Battery Volts display module and Light switches removed from the panel on the side of the fridge.
See image of the PCB below.
This is available to order from Prima leisure Part number1000203 for £130.98
The relays are on the rear of the PCB mounted fuses you can see in the consumer unit below the fridge.
Relays A, B and C are the habitation relays. They are only energised when the 13 pin plug is connected to the car.
A & C only when the car engine is running
B is energised as soon as the caravan is connected to the car.
Relay D is closed by the power switch
Relay E is closed by the internal lights switch
Relay F is closed by the pump switch
Unicorn 3 Printed Circuit Board
Fuse 13 is the Van battery 20 amp fuse and is viewed from the rear