An annual damp test should be carried out, either diy or additional to the annual service. A damp meter is needed, calibrated in % not just green/red lights.
Test internally, in about 20 places, near the external seams ie front, top and back and round the windows/skylights. Anything up to 20% is an acceptable reading to account for the natural moisture in wall boards.
Using a good wax polish, worked into all external seams will help to keep it damp-free but in reality we're all in the hands of the original caravan manufacturer as they sometimes save a