This could be caused by water penetration leading to black mould in the timber frame. Seal the outer joints with silicone. Check and seal any penetrations through the shell such as lights, roof openings front and back with rubberised mastic as required. Then start taking the window surround rubbers off to check for rotten timber which should be cut out carefully and replaced before re-sealing the rubber outer joint.
On the other hand if your van has a shower fitted, the problem may be more serious and apart from taking it out and checking the floor and replacing the damaged area the sealing everywhere with mastic, I don't no what else to look for.......
stan the man, July 2009
