The idea is to get something that won't absorb a lot of moisture and will dry out fast.
This type of shower curtain is often sold as a liner but is fine to use alone. They can be washed with some bleach and look good as new, and last for a long time. I wash mine (at home) about every 3 weeks.
Microfiber Shower Curtain LinerOf course they do get wet and have to dry off. To that end, you need to run the bathroom vent fan for a good amount of time after showering. I also keep a dedicated towel for wiping down the shower walls and floor, wring it out in the sink, and then hang it outside to dry. That removes a good bit of moisture from the RV.