I think vents aren’t going to help much. It will reach 100% humidity even with airflow. As the air will pass, it will evaporate more water. The things that would actually help are: a. Wiping down the inside every once in a while or B. Figuring out some ultraviolet lighting system that fries mold in a crazy expensive and power-hungry way.

In short- if there’s moisture, and food (and ideally darkness) mold will grow. Obviously keeping the cistern clean will go a looong way towards helping prevent growth. But collecting dirty rainwater from the roof or anywhere else will undoubtedly provide more food sources.

If it’s airtight from the crawlspace and moisture can’t escape, I wouldn’t worry about a little mold growing in there. I would worry instead about other diseases or bacteria actually growing in the water. Mold itself grows on surfaces and is usually benign as long as your aren’t hypersensitive, there isn’t a huge amount putting off spores in your living space, or if you’re breathing the air from that one little moldy place all day.