Hello John,
I'm presumming you don't have experience with using moisture meters otherwise you would know what to get.
When taking moisture readings, you cannot rely on the readout alone, you have to know what you are looking at and how to interpret the results, especially with caravan construction. The wall boards can gave extraneous readings, becasue of the vinyl coating, and understanding what is a good/bad reading for different types of material is not always straight forward..
If you are looking for accuracy, then you must find one that comes with a calibration certificate produced by an acredited laboritory. that is expensive, and why its usually dedicated professionals that have them.
For anything without a calibration cert, you simply do not know its accuracy, and just becasue one person has one that seem fine does not guarantee the next one on the shelf has the same performance.
You pay your money and for most you take your chance.