Regarding your wet tank sludge:
It is normal to drain the air brake system daily (when driven). If this has not been done lately, there may/will be quite a lot of goop in there. The goop is water (moisture from the air) and oil (from the engine -- blowby ("reverse blowby") in the compressor). Nothing to worry about, so long as you keep it drained.
The basic air brake system has two tanks in series. The first tank after the compressor is called the primary or wet tank, because that's where most of the moisture settles out. The secondary tank then supplies the less contaminated air to the rest of the vehicle. There is a check valve between the primary tank and the secondary tank, so that air will not be lost "rearward" in the system if a breach occurs in the wet tank.
Many vehicles have more than two tanks. Millicent has four, but it looks like two at first glance. Each of those two has a divider in the middle, making each effectively two tanks. No, I have never bothered to learn exactly how the four interact.
I recommend pull cord drains on all tanks. Four bucks a piece at any truck parts store. I rigged mine so they are easy to reach, and I give them all a tug each time I drive the bus.
You are looking at four little steel cables that I ran thru holes in the bottom rub rail. Each has a loop on the end for tugging on, and the other end is attached to a spring loaded drain valve. Makes it really easy to keep the tanks drained.