Fresh water tank drained with the grey tank

[chay1950]

I had the fresh water tank filled to capacity. The water was at the very top. It's located under the sink on my 2002 2720SL. When I opened the grey water valve out side the trailer, the grey water drained and then the fresh water tank drained. This is the first time this has occurred.
Before, the fresh water tank was 3/4 full at most and never drained. Any ideas about this?
I haven't been able to look at it as my back muscles have cramped up and it's very hard to get around right now.