Time liquified the clog to the point that it flushed, but I was greeted by the bowl filling with black anaerobic stink water after I stopped the macerator motor.
That's a very strong indication of blocked tank vent.
Given the uncertainty about the holding tank level, it seems prudent to pump out before doing much more.
Attempting to pump out will reveal whether the tank vent is blocked. If it is, the pumpout will only be able to pull out a couple of gallons before it pulls a vacuum that prevents it from removing any more. So pay very close attention to the sight glass in the pumpout line to se how long anything is moving through it. If very little, cease pumpout immediately to avoid letting the suction implode your tank. Also have someone paying close attention to both your toilets. If they hear any hissing in one or both of them during pumpout, that's a sure sign of a blocked tank vent...the tank is trying to pull in air from any source it can find.
I have no idea what that orange thing is...I don't think it's a vent line filter 'cuz no hose exits from it (or maybe enters it...impossible to tell what the direction of flow is) but I can see that there's a kink where it dips behind the white hose and that a black hose is teed into it that wanders off across the ceiling to disappear into a manifold connecting two larger diameter hoses.
I'm more interested in knowing what's at the other end of the hose that runs under that hose and terminates in a fitting in the bulkhead.
--Peggie