Moisture?

1999 shown book price £4,000 about right insuring with Motorhome/Caravan club. Big mistake not getting damp test and full operational appliance test.

room,water heater and fridge don't light on gas.
Suspect damp when fixing up shelf where folding fifth bed (removed)was, either one screw missed framing in wall or wood rotten. Just our luck dealership under new management a month after buying,maybe to avoid court cases of others.
Question: Are the cheap moisture meters reliable £11