On my TH TT where all tanks are exposed, has FW tanks that are black in color. We can and do carry in excess of 100 gallons of fresh water depending on the type of RVing we are planning to do, we like that it has two black colored fresh water tanks as it does not allow light to penetrate, zero issues with mold in freshwater tanks over a dozen years of use.
At times fresh water in tanks can sit for months at a time, again no issues with algae growth. We learned the hard way to bring fresh water from home for extended boondocking or dry camping, ran into poor quality water on a couple of occasions trying to fill up on the way, no issues since we specked both the trailer and the tow vehicle to handle the nearly 1K# of extra fluids we carry when necessary.
