I recommend not relying on the indicators, just as a guide. The indicators in my 3200BHT have read 3/4 full since we brought it home new from the dealer. When it drains at first, it'll read empty and then one flush read 3/4 full and then full. I leave my valves closed and drain it every other day while staying at the CG. I then drain it and use the black tank flush as well. I run that a few times until the water comes out "clear" and then eventually stops.
Why do you think it's not emptying 100% outside of the indicators?
2013 Prime Time Tracer 3200BHT
2016 GMC Crew Cab Duramax All Terrain
Ribs in the smoker and Whiskey in hand