Look into a product called Wet n Forget.
You will most likely need the one for exterior surfaces. I've used this on the Airstream and it works well. It takes several days to several weeks to do it's thing, but it does absolutely work on mold, mildew and fungus. It will then leave a slight residual on the skin of the camper and continue to kill mold, etc, for another year or so. It's one of those products that's almost too good to be true.
You can even call the company and talk a live human being in the United States who can help you choose the right product and the best way to use it. (for the siding on my house, I just spray it on and wait a couple weeks, mold and mildew are gone)
I think your camper has a fiberglass shell? What is that belt-line area made of? Aluminum? This product should be 100% safe on both, but check with them to be sure.