If you let the water run into the tank, it will eventually back up coming out the fill hole. There must be either a overflow, or a vent. The tank has to be able to get rid of the air some how when it's being filled with water.
On our rig, there is a small screen, about the size of a dime. It sits right along side of the fill hole. That is the vent. When I'm filling with water I can hear the air being expelled out that vent. If I get it over full, water will come back out that screen, plus the fill pipe.
2008 Cruiser CF26RK
2013 Ford Eco-Boost
Heavy duty payload - Max tow