It's been said you do need to know a few fundamental things or you fool yourself with a damp meter.
First the sort of weather, if it's damp and you simply place the probes on the surface you may well get a reading.
For the same reason if you take a cold van into a warm workshop and straight away give it a damp check, all the reading will be sky high due to warm moist air condensing on the vans cold surfaces. (this little 'stunt' has to my knowledge been used by one dealer checking a trade-in!)
Surface moisture though is not important, it's what's underneath that matters, so deduct any surface reading from any higher reading after the probes are pushed through the surface.
Having said that, given this van has alleged 30% to 50% readings, I'd expect to find plenty of evidence without the use of a meter. It cannot go from 0 to 50% in various paces and in only a few months, therefore the structure will have suffered.
Soft areas, blebs or other tell tale signs should be clearly evident to collaborate the meter readings.
My thumb can find the common damp areas at a lot less than 30% and often without even setting foot inside?!
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