Damp meters are rather like statistics, they can be interpreted in a number of different ways, and unless you are knowledgeable about their application you can easily be miss led by their readings.
Basically the meter reads the electrical resistance between the two probes. The probes should be fixed distance apart, and when used should impress their shape into the surface being measured by a known amount, thus giving consistent readings. Unfortunately most commercially available meters are designed for use in brick/stone built buildings, and unless they supply a chart of values for use on thin walled pressed composite panels (such as those used in caravan wall construction) their readings are not to be relied on.
There are other factors that will affect the readings you can take in a caravan, the relative humidity, the temperature of the panel, any condensation present, and additional sheeting such as aluminium foil on the hidden side of the panel.
So whilst you may wish to take percentage readings, you must also have the necessary information and skills to interpret the readings. For the reasons above I do not trust a blanket statement saying your caravan is damp, I also insist on seeing the method and interpolation of the results ( not that I fully understand them, but it frightens the dealers when they know they are dealing with some one with a better understanding than them their own on the subject.