[rdlamb]

Ouch:
Dan, block up the wheels on the opposite side of the coach from the waste gates. That will "dump" more of the holding tanks contents toward the exit. If that produces a lot of results, it means GS drilled the exit pipe to high into the holding tank, and you will always have a certain amount of stuff sloshing around in the tank.
It also means lots of continual smells, and other problems, especially if you store the coach through a winter where the contents may freeze solid.

If not much comes out and the sensor still records 1/3 full, the sensor has crud on it and requires a blasting with a holding tank wand. The backflush systems usually do not spew water high enough to clean the sensors, rather they shoot water across the tank's floor to move stuff along.

I can get mine to work by using a holding tank wand and a lot of tank washing. In the end, I just gave up and my sensors are stuck on 2/3 full, even when the tank is empty.