Mine did the same thing. I just bought it this year (97 Allegro Bus) and took it to be dewinterized on June 25th. Got a call from my mother's hospice nurse while we were at the dealer waiting that she had taken a turn for the worse so they hurried up so we could get out of there and when we got home and got out we noticed what appeared to be fresh water pouring out onto the ground. We couldn't deal with it then because we had to get to the hospital.
The water appeared to be running out of a short plastic tit/pipe kind of thing on the tank. Should there have been a cap on that?
A few days later when I had the chance I checked the monitor and the fresh tank read close to empty.
We've just finished clearing out my mother's place and catching up on yard work around here so I'll get back into the MH in the next week or so. Thanks