As Tony points out, the method is not accurate for a number of reasons,
How far do you press the probes in, and how far apart,
Given a reading what do you make of it.
No wooden surface is 100% dry so you will get some reading. The conditions such as humidity, and temperature, the type of surface you are testing, along with any surface covering will affect the reading.
You would need to be a real expert to be able to interpret the reading you get
Do not rely on it as method of establishing the moisture content of your caravans surfaces.