Good work and an Interesting write-up.
I've had large boats for many years with aluminum, s/s, plastic and fiberglass tanks. They all had one thing in common, a 6" inspection and clean out hatch. So I've looked in a lot of tanks over the years. What I've noted is that mold starts to grow on the top surfaces when the tank's not full. The black debris you see on the bottom is likely the mold clumped up after its' been flooded with a fill up.
The nice thing about the clean out is that you can physically scrub the interior and get it spotless. While we have always normally had a separate fresh water supply for drinking, we have used tank water for everything else, including brushing teeth and have never had a problem. The way that I could tell that the tank needed cleaning was when tea made from it didn't taste all that good.
For the trailer we do the same as most but we do always add a touch of bleach each time we fill. So far the tea still tastes good.