Can we travel with fresh tank full? We lose half on the way.

I had a similar issue with losing a lot of water after filling up. We were going to a campground in Utah that only had electric, and of course it was at a higher elevation than where we started so I think the tilting of the TT lost us some water. So even if your vent is working properly, the natural motion of the TT, including going up and down hills, may be enough to lose some water.

In any event, I installed a valve on the overflow as suggested above and no problems since.