There are many designs for shower curtains, but there are basically two types. One is the plastic type that costs about $1.95 from Dollar General, and you can find them anywhere. The other type is the "cloth" material type curtains. They are indeed made of fabric, but are treated with a water resistant treatment that prevents water from penetrating. They usually cost a bit more, can still be found in most places, and come in all kinds of colors and designs.
As with any shower curtain, either one will hold water drops until the water drips down (cloth or plastic). There is always a residue of water left until it evaporates. That evaporated water is now in the air, and yes, it will settle somewhere else eventually, sometimes as rain, sometimes as dew, or in your camper as condensation.
But, with either style of curtain, if you are using your camper in a normal fashion, there is almost always an influx of fresh air and an out-flux of (stale) air. If you are using your camper in a normal fashion, in the Summer, roof vents open, doors open and close, windows open and close, fans blowing, appliances running, the water evaporates from the curtain, just like in a house and has no or very little effect.
Yes, you do need to wipe up any excess water, dry the shower / bathtub, or use a squeegie on the walls to help get rid of as much water as possible. But with just a bare minimal attention, you'll never have mold or black spots.
If using the camper completely closed up for a long time, then yes, moisture gets trapped without any kind of ventelation, and yes, mold growth has a potential be more prominent. But even in the winter, with moderate care and attention, wiping up spills, water spots, wiping down the shower/tub helps go a long way keeping "black" off the walls and around the molding.
Use any kind of curtain you want. You can always take them and shake them outside, toss them in the laundry to help keep them clean. And the nice thing about the cheap-o plastic ones? They don't cost much, they never last long anyway (yes, they all do begin to look nasty after a year or so, even in your house), so the el-cheap-o plastic $1.99 curtains are easy to just swap out any time.
My previous 2 campers had nothing but plastic shower curtain, and we never had a mold or "black" problem. But we did change shower curtains every spring (and the old ones became paint, stain, and varnish drip cloths in my wood working shop.
