Lots of good advise in this thread. I wouldn't trust the tank level indicators as they are notoriously unreliable. We're on our fourth fifth (that doesn't sound right but it is technically accurate) and all of the indicators in each of them were an estimate at best. I can't really comment on our latest one because we've only used it once but our previoust one did exactly what you said yours did i.e. showed 1/3 full after dumping but a couple of hours later showed empty. After you've used your rig a few times you'll get to know pretty well how accurate they are.
Dump station etiquette is something that you were obviously concerned about. Many people are not that considerate. Good on you but remember that you have as much right to be there as anyone. Just do what you have to do and then move on. That's all anyone can reasonably ask.
Remember, good judgement comes from experience and experience frequently comes from bad judgement. Learn from the mistakes of others, you won't live long enough to make them all yourself.