Most leveling systems have a sensor in the reservoir for the leveling jacks to determine if all of the jacks are retracted. It works by expecting that the fluid be a certain level if all jacks are fully retracted and if the fluid is not at that level then one of the jacks must still be down.
Its important to note that rocks will sometimes get lodged between the foot and the “RAM” of the jack which will keep the jack from fully retracting. Before you add fluid verify that all jacks have fully retracted. There should be no gap (Shiny) showing between the foot and the jack. If there is this will cause the system to think a Jack is still extended because there is still fluid in the jack that didn’t fully retract thus the reservoir will show low on fluid when its really not low.
The following graphics show where to check for low fluid depending on if your system is by Power Gear or HWH.