Yes we need the year, model name and number to give you a correct answer.
As for the rest it appears that when you fill your fresh water tank until it overflows it drains down to 3/4 full which is what many of them will do since once it starts overflowing it will start to siphon water out due to the way the trap to discourage insects from getting into the fresh water tank is designed.
Note that if you go to Winnebago's web site using the following link you can download the owners manual for you coach and find the answers to many questions about how things work:
https://winnebagoind.com/resources/m...tormanuals.php