We've learned that you cannot trust the gauges!
I was just re-reading one of my Trailer Life magazines and found an article on this very subject. It is in the January 2005 issue if you're interested. I was surprised to learn that even the fresh water tank gauge will get corroded and not give and accurate reading.
Basically the article says that the RV industry uses the internal type gauges to help keep down costs. But there are after market units that mount on the outside of the tank. They supposedly are very accurate and will never get corroded. Cost is in the $200 to $300 range.
Like many of you we have learned about how long we can go before we need to dump. A few trips and you'll get the feel and smell of it!
HamRad