I would loose a little water until I fitted my trailer with a better gravity fill connection with locking door. The old one was like the shore power mouse hole with a trough and vent hole behind the cover flap. The newer one is recessed with a cap on the gravity fill and the vent all behind a lockable door. When traveling with water in the tank I also insert a small cork I got at Lowe's from the miscellaneous hardware trays into the vent hole inside the door. However the main reason I replaced the mouse hole type water fill was due to rain water & dirt getting inside the tank.
