RV tanks are designed to turn everything in the tank into a liquid. The RV’s starting and stopping motion breaks down the solids and mix it with the flush water so the contents in the tank becomes a liquid. This liquid will then be able to flow out of the discharge value(s). The tank is also vented which helps prevent implosions when hooked up to a vacuum source.
When we pump out an RV, we attach our hose to the discharge valve(s) and vacuum all the waste that has turned into liquid and that flows out of the valve. We service both grey water and black water holding tanks.
There are several factors where this situation could happen. If the RV has been in heavy use but has also been sitting still for a while, the solids will build up, especially under the toilet. These solids will not flow to the discharge value and would give the appearance that the tank was not pumped (even though it was). Also, if the tank gets full, solids can cover the gauges sensors which will show a false reading.
The best way to prevent this is to either drive the RV around to mix up solids in the liquids before we arrive or while we are on-site you can get a garden hose and spray water into the toilet to break-up and wash away the solid build-up as we pump.