As stated above, somewhere on your camper the fresh water tank has an air vent. My previous TT all had gravity fill ports with an small screened vent tube right beside the fill port. When tank was fill, it spitted out that vent tube.
On my new 5er, the fill is located in the front bay area, and the water tank must be located in the rear. On the side of the 5er is a little black plastic thing. When the tank is full, water comes spewing out of it. That's my air vent tube. Fill yours, when water spews out of a tube or a funny looking gizmo on the side of your camper, you know the tank is full.
And, the end of that tube will be set up higher than your water tank so no water sitting in it can cause a vacuum. So, it will be located somewhere around chest high on the side of your camper.