Keith wrote:
You need to figure out
where the moisture is coming from that allows the mold to grow.
Probably lack of ventilation, or condensation if you're in a cold
climate.

I would guess is it a leak around the rub rail due to the location of
the mold. If it is Black Mold per Tom's post, it has to be due to a
long period of dampness. Black Mold is toxic and needs to be handled
with care, weat a mask, ventilate the area with a fan, and after
cleaning the mold, wipe down the rest of the inside of the cabin.