There is mold killer stuff that's sold at paint stores. If you don't
want to use bleach, try that. It doesn't take much bleach, try a very
dilute solution. Lysol will do it as well. 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. Vinegar should also do the trick, while not the best smelling
solution.

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.