Are you relying on the electronic level gauge to tell you how full the tank is? If so, ignore the gauge. They are not accurate, specifically the black tank. As soon as TP touches one of the gauge sensors, it closes the circuit and tells you it's full.
I have a 37 gal black tank and can go about one week with 4 people before I need to dump, and we use it a lot.
If your toilet pipe is clogged, you likely have the well known poo pyramid forming at the bottom. This is generally caused by not having enough liquid in the tank to start.
After dumping I always add about 3 gals of clean water before using the toilet. Today's holding tanks are only abt 6 inches high and very wide. Water is need to disperse the solids so pile doesn't Form on the bottom