I agree, I've bought most of my vans that didn't have a recent one, and I bought a damp meter, cost can make it appear that more is better but I bought a cheap one and it's been fine, I do a test on a known dry piece of wood that day, and note the reading, test the van in the most likely places for damp such as around exterior locker doors and windows, the hab door etc and if they are all within a few percent then either the van is very damp or very dry, basically I then compare readings all over the van and if you have a lot of highs then the chances are you have damp, all it is is a dedicated multimeter with defined parameters.
Other people may disagree with that method, but out of 11 vans I've only been fooled once.
Of course you may have a very new one and it is part of the warranty, in which case we're back to square one I'm afraid.