Damp in a caravan can show through extremely quickly and does not by any means indicate that damage has, or is likely to have already been done.
I had a repair done on my caravan (bodged) by a dealer and when I tested it, the area was bone dry. After just one or two day's rain, the meter showed damp and I was able to go back and have it rectified immediately.
Thereafter, the meter was invaluable in monitoring the drying period, which took several weeks even after the small leak had been properly repaired.
I now use the meter about once every other month and if nothing else it either reassures me that there are no more leaks, or alerts me to possible problems. It is true that you have to use common sense when using it e.g. if there is a lot of condensation about, but it isn't rocket science.