Water leaking out of vent pipe while traveling

[bbrown2180]

I have a 2021 Montana High Country 945RL 5th wheel. Twice I have filled my water tank full to go hunting and my water tank is only a third full if that much when I get to my dry camp. While turning my rv around in the driveway I have seen water sloshing out the vent cap. With a full tank sitting at home it doesn't leak, only when traveling. The dealer says they can't find anything wrong. They haven't opened up the underbody to check, they say if they do that and can't find anything wrong the warranty won't cover the labor charges. But yet there is something wrong, I shouldn't be losing all my water while traveling. Any help or ideas would be apprieciated.