I have a similar moisture meter I use on fire wood, not sure I would want to use it to take a measurement in any part of the van that I didn't already know was completely rotten and had to be replaced.
There is a pair of pretty robust metal probes that will leave a significant permanent mark on any surface you engage it properly with.
If you think it's rotten, it probably is, so replace it. These things are useful if you're buying firewood or if you're determining if there's any hope of salvaging something in a restoration. You won't be using it on an inspection before buying.