Can anyone recommend a damp/moisture meter similar to the sort they use in caravan/motorhome dealership to test for water leaks?
Is it you buying a van , if so don't buy a dear wet one
Dehumidifiers from £30
As said the cheap ones (including the Aldi/Lidl ones) use two needle prongs but there are cheap non-invasive ones from China available. No experience of them myself.
4-in-1 Inductive Wood Tree Timber Moisture Meter Drywall Masonry Non-Invasive | eBay