Damp meter

Think you will find the green, amber, red meters also have metal prongs

When they checked our Laika a couple of months ago they used one that had a smooth base that you just ran over the surface like a large computer mouse. I watched the guy using it and he commented that people with expensive Motorhomes don't like finding holes where the probes had been. They also work better on ceilings that are covered with fabric to help with insulation. Very quick to use and he checked the whole of the ceiling which you cannot do with probes.

Like this?

Protimeter

Yes that sort of thing but with 3 coloured leds instead of the % reading.

With the led ones I would be curious to know when it switches from green to amber, I would assume around the 20% mark.

I have a % one with probes that I use to check wood before using on the logburner. As mentioned one does not really want to go around stabbing the van any more than is needed.