The problem is most likely your probes. A film develops on them and causes them to sense water where none exists. You can take them out and clean them but the problem could also be a bit of toilet paper over the end of the probe causing them to once again, sense water where none exists.
On an older camper I had I changed the probes out. System worked perfectly til the film migrated down onto the probe from the wall of the black tank and once again, it was always 1/3 full.
I think the definitive answer is to change the probes but use the Valterra probes. They come with a shield on the top to protect the probe from toilet paper and the film sliding down the tank.
Here is a link to an eBay auction showing the probes. Im not advocating that you buy these, SHOP AROUND for them but they have a good photo of the probes. It IS NOT my auction either.
Valterra T21301VP Tank Probe Black Water 4 Pack | eBay
I know I protest too much but I don't want someone to accuse me of trying to sell you something when it is not true.