Swiftman, having searched the past responses relating to damp readings I found one, which suggests that 8 to 12% is normal , with 15% being acceptable.
As vans are made of wood, in various forms, there is bound to be some kind of damp reading, and also it depends what kind of day the readings are taken on.
A high humidity on the day will produce a higher reading, as would if the readings were taken immediately after the van had been occupied for a time.
I would suggest you contact the manufacturers and ask their opinion.