"holding" is your Black tank (the toilet). When you have 2 gray tanks, most typically, one of them is the shower and bathroom sink, the other is the "galley" (kitchen) sink.
If you have 4 drain valves, 1 is your black tank, 2 are for the gray tanks, and the last one is probably your fresh water tank.
It's very common for the level indicators to not read "empty" even when the tank is empty, either because they are malfunctioning, or there is "matter" on the tank walls that is causing the false positive.
There are a few different techniques for flushing/cleaning the tanks which will sometimes get the level sensors reading correctly. Does you trailer have a "black tank flush" hose connection in the convenience center (where you fill the fresh water and such)?
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2002 Ford F350 7.3 Super Duty XLT 4x4 CC LB SRW -- Pullrite Super 5th 16K
2017 Keystone Carbon 337
2022 Honda CRF-450R
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