A company from Canada recently managed to get a post up on the forum, but becasue it was advertising it was quickly taken down, but not before I had the opportunity to look at it.
The company were suggesting that water ingress problems could be detected by using an infrared thermal; imaging camera. It was designed for buildings, but they were suggesting it was also appropriate for RV's. and it was depicted as looking at the outside of an RV.
Its my opinion that based on the fact that TIC (Thermal imaging Camera's) which display a colour picture where the hue is analogous to the temperature of the object or surface, if the damp patch were the same temperature are the rest of the surrounding material, then it would not show up as being different.
However if a wall was damp and the caravans was being heated up, the dampness in the wall would not heat up so quickly and thus it would appear to be cooler, BUT there are so many other reasons why certain parts of a perfectly dry caravan would heat up less quickly, for example most heating systems cannot raise the ensure inside of a caravan perfectly evenly. There could be a frame or another piece of furniture or soft furnishing that keeps part of an exterior wall less accessible to the effect of internal heating.
Also the trend towards wall constructions where there are no wood frames or the insulations is non absorbent would prevent any moisture present from significantly causing localised temperature difference.
As with the more conventional moisture meters being discussed in this thread, it would require careful usage, and a competent user who knows how to read the systems output and allow for both environmental and constructional differences to interpret the presence or other wise of worrying levels of damp.
So I think at best the IR camera would be very poor identifier of damp areas, and it certainly wouldn't show where the source of a leak was. And at worst it would be quite expensive toy with limited applications.