Yes, you are correct that there is no 'Acceptable level' , despite what people on the internet discussion boards post. Read a post fro a dealer who does not take action unless its over 30%, been working that way for years and has never had a claim a leak was not detected. There is money to be made doing work that is not needed.
Here a commonly used guide.
Readings between (0-15%) no cause for concern
Readings between (15-20%) dealer to make a note of these readings and re-check at next service.
Readings between (20-24%) possible early signs of ingress, look for tangible evidence or ask for a re-check.
Readings between (25-30%) moisture evident, remedial work required; may not need stripdown unless surface damage (staining, pimpling, and softness) is apparent. (There is a risk of wallboard deterioration due to retained moisture in the structure if resealed only.)
Reading (31 % and above) structural damage is occurring, deterioration inevitable. a full stripdown of the affected area is required.
But this is an over simplification. The moisture content of wood (and other porous materials) is determined by the humidify (which is affected by temperature) water contact and past moisture levels. After a hot Canterbury summer it my drop below 10%, but a damp Northland Winter will easily see it close to 20% . Living in a caravan increase humidity so I would not be surpirsed to see a lived in caravan approaching 20% (apparently danger levels). The problem is not dampness caused by living it, its leaks. Leaks will be localized (unless its leaking everywhere), so you are looking for spots with higher readings, not the reading itself.
The way I use my moisture meter is A-B tests. I get 13 here, and 12 on the other side, looks good. I get 25 here , but the same spot in the other three corners reads 25 - either I have 4 leaks, or something about those corners makes it read high.
Don't be alarmed when you see me talking to myself, I am getting expert advice.