On my black tank I could never get it to read empty .always read 1/3 and no matter what I did the same always was there.
But one day I was at the dealer looking to see if there was anything new to try to get this to work better and this tech was standing there so I asked him.
He told me the best thing that works for him was to after the tank has been drained leave the valve open remove the cap and let it air out for a few days .
It worked!!!! So now I do this everytime I'm home between trips.!!!!!
The next time Im on a trip I will try to drain the tank and leave it open for the day and see if that works too. I will also put a note on the bathroom door to put hose on so no mishaps will happen !!!!!!! Lol