I would have to agree that it is some kind of moisture problem and some form of mold. A mixture of bleach and water should kill it, bu tif there is a moisture problem it will be back. Time to become a detective and find out the why this is happening. Take precautions when dealing with mold, you don't want to breathe the spores, wear gloves.
About this irv2 or forum
Someone found out a whole batch of untreated wood caused massive green fungus inside without moisture even
Manufacturers took coach back I think as was a whole batch of rvs infected