Depending on how many tanks you have and how many indicators you have on the panel, there may be a wire that is not used.
As an example: let's say your panel "shows":
1 fresh water tank
2 gray water tanks
2 black tanks
And your unit has:
1 fresh tank
2 gray tanks
1 black tank.
In the above example the monitor panel will have 7 wires and only 6 will be used. If any tank only reads 1/3 full, the issue is more than likely either the wires fell off or are loose AT THE TANK! Or the sensor pack AT THE TANK has failed. Have you filled the black tank to the point it is coming up the pipe at the toilet?
I would disregard the green wire for now, the coroplast needs to come down and the tank accessed. If you can do that, make sure which tank is which by following the drains back. As a rule black tanks have a 3" outlet and grays have a 1 1/2" or 2" outlet, but I have seen 3" on grays.
Once you have access to the black tank, locate the side with the sensors and wires attached. The bottom one is ground. Try jumping from that sensor to the frame, if that fixes it just put another ground on. If not then start jumering from that sensor to the other ones going up the tank. If jumpering doesn't indicate, then the sensor pack is probably bad. If it does then the sensors in the tank are probably bad. Either way is an easy fix.
If this is something you want to attempt, I'll be more than happy to give you further instructions.
Just some added info: if the sensor packs and monitor panel are KIB, the jumper test won't work on the FRESH tank. You have to use a resistor ( can't remember how many ohms) in the jumper as they have changed the boards to accommodate minerals in the fresh water.
Are you confused now?
2012 Copper Canyon 273FWRET being towed by a 1994 Ford F350 CC,LB,Dually diesel.
Airlift 5000 bags, Prodigy brake control, 5 gauges on the pillar.Used to tow a '97 Jayco 323RKS.
Now an RVIA registered tech. Retired from Law enforcement in 2008 after 25+ yrs.