It is not uncommon for wiring around 240V cabling to pickup some voltage from induction. In AV, one quickly learns to run microphone cable well away from lighting cable unless one particularly wants a 50Hz or 100 Hz hum in the speakers throughout the performance.
Yes, you may well be able to pick up some AC induced into the 12V cables. Especially if you are using a digital multimeter or a very high impedance analog multimeter.
So it could be induction OR it could be leakage from a 240V device such as the Setec.
Now, if it was leakage, the RCD unit would pick it up and trip the circuit. You need to check that this is indeed functioning correctly.
First test, go to the RCD unit and press the test button. It should trip the breaker. Reset the breaker and test it a few times.
If it trips every time, there is a strong possibility that it is not leakage from a 240V appliance.
As a reminder, this should be checked EVERY time the van is connected to 240V to ensure the occupants are protected.
But to make absolutely sure, unplug all appliances from the sockets except the Setec : stove, fridge, H/W etc to see if it is any of those appliances.
Check for the AC voltage each time.
Finally, unplug the 240V lead for the Setec and check again.
If you have had AC still on the 2V wiring after all this, it is most likely induction from the 240V cabling into the 12V cabling.
As I said in a previous post, measure the resistance from the earth pin on the 12V trailer plug, pin 3 to the van drawbar and post your results.
It should be close to a short circuit. If it is not, the van -VE is not grounded to the chassis. If that is the case run a wire from the -ve of your battery to the chassis and the voltage may well disappear.
Another question. Are you making these measurements with the van disconnected from the vehicle?
If so, you will probably find that the voltage disappears when you hitch the van up and connect the 12-pin plug. In that case, the car earth will then ground the van and the problem may disappear.
Again, if this is the case, your van -ve is not connected to the van chassis except through the car.
If you are in ANY doubt how to follow and understand the above instruction, go and get it looked at professionally. Faulty 240V wiring inside a metal van can be dangerous and can easily kill someone.
take care
Mike