[Steveo57]

If the overflow line runs for any distance to the tank it may have a high point that can form a loop seal. That can prevent the tank from venting after you put enough water in. I don't know how your tank is situated with regards to the overflow point but it's something I've heard of happening to others.

One case I remember was when the tank was located in the rear of the trailer and the overflow was towards the front. The vent line ran along the frame and it had a high point which when the line got filled up it the tank couldn't vent and the guy couldn't get any more water to go in.