Just had our annual service
Damp... 40-50% on the off side near the top
Garage says rail needs to come off and be resealed
Then dehumidifier / heater constantly on will eventually bring this down to a reasonable level
There are NO signs of damp from inside the van, no soft wallboards, no smell, nothing at all .....
Not sure what to do as I know lots of people say we should dissect the van and investigate - really not keen on that as we are not DIY competent !