Hello Bob and Chris
Most of these meters are designed to clamp around just the live meter tail. they work by sensing the magnetic field generated when a current flows in a conductor. If the device surrounds bot live and neutral wires (as in a power cord) the current in each conductor is in opposite directions and thus will cancel out.
In a caravan there are no exposed individual tails as in a domestic installation, thus the device may not work.
However. a few years ago I did have a meter that did manage to detect AC current with a clamp around the three core supply cable and it also measure DC currents with a non-contact method, but I have never seen another since.