Hello Sarah,
You are right to be concerned about damp, but as Klarky says, cheap meters are likely to give you misleading information.
The theory is that any dampness in the structure reduces the electrical resistance of the material. The meter basically measures that resistance.
This may seem relatively simple but the picture is complicated by the fact that different materials have an inherent level of resistance, and that fact may lead you to suspect dampness, when in fact the actual level of moisture is quite normal.
Other factors that affect the reading will be temperature, and the level of humidity, salts and chemicals used in the construction of the wall or application of wall coverings.
For a moisture reading to have any real authority, the meter must be calibrated, and the user must be trained and have the relevant information about wall construction and their inherent resistivity to be able to interpret the readings.
I seriously doubt whether most caravan dealers are actually fully competent to survey a caravan for dampness and to give an authoritative report. For exactly the same reasons it is unlikely that an inexperienced person using an uncalibrated meter would be able to make a fully objective assessment.