Damp floor on a Bailey Ranger

Dudley Walker

Has anybody had a problem with damp patches on the underside of the floor.on a Bailey Ranger. No problems on the floor or walls inside. They are just each side of the wheel arches when the van is stationary in the garden.We can't find where it is coming from It's not much be I don't want the ply on the underside to rot. Any help would be appreciated