Blackwater holding tanks, how are they set up?

[Bob Quinn]

I have a 1980 53 MY, it has two holding tanks.
Previous owner installed what appear to be 2 dometic lights, I am assuming one for each tank.
I was told by the mechanic that they would turn yellow when almost full. So we were on the boat about 10 days and I noticed one of the lights came on red, not yellow. So I am assuming that was just an error on the mechanics part and he meant they would turn red.
Anyways, it was the upper of the two lights so I figured that would be to the tank that I assumed serviced the forward head. So we said o.k. let's just use the master stateroom head until we get to the pump out. Then I saw we were pumping overboard from a thru hull fitting toward the front of the boat starboard side when flushing the master stateroom toilet.

So my big question is: can anyone tell me how these systems are plumbed together?
I would have thought the other sensor light would come on well before anything begins pumping overboard.

We did do a pumpout and the light went out and everything went back to working as it should.
Now I see the same red light is on about a week later, again the lower light sensor has never lit up.

The toilets are Raritan brand.

Hopefully someone could share how these setups work?
Thank you!

[MadHatter53]

Mine is set up for the owners and mid head to go to forward tank the forward head goes to mid/aft tank.

[SKYCHENEY]

Mine is set up for the owners and mid head to go to forward tank the forward head goes to mid/aft tank.

That's odd. The original setup was fwd and aft head go to the bow tank and mid head goes to the stbd side tank in the gen room