[DutchmenSport]

The over flow is actually the tank vent. If you attempt to fill from there, first, the tube is only about 1/2 inch or less. Second, if filling there you'll immediately have a vacuum and nothing will go in.

The old gravity fill ports had a large tube from the port to the thank. Then there was a second, much smaller tube, that ran from the top of the tank, back to the outside of the camper, usually right next to the fill port. When the tank got full, sometimes those vents would belch water up. But the gravity fill ports worked only because the tank had another vent.

I've never understood either, how anyone could use the vent as a filling location because of it creating a vacuum.

I understand filling from the tank drain. It drains from the bottom. That vent is at the top and will let the air escape as water fills from the bottom. Close off completely that vent, and even filling from the bottom, a vacuum will form quickly. To much pressure and the tank will break open or explode.