Do not just shove the hose in the fitting. It largely blocks it and prevents air from escaping. The vent you circled will vent air as the tank fills but my experience with filling tanks is the air cannot escape fast enough, and you build pressure in the tank.
On my Thor Trailer, I nearly burst the tank (but didn't realize it till much later) and when I removed the hose, water gushed out of the filler as the tank contracted back to normal size. With the Bigfoot trailer, I again got stupid and shoved the hose in the filler. I almost immediately began hearing water inside the trailer and pulled the hose out and opened the compartment door that exposed that area. I ended up spilling a gallon or so and rapidly mopped it up with some towels and other cloths. The pressure had caused the filler tube to the tank to pop off the filler adapter in the side of the trailer. Another Bigfoot owner did the same thing but his tank swelled up until it broke some of the metal supports. It was a mess to repair.
I looked in the Bigfoot manual (such as it is) and did find that they call for the use of a filler adapter, such as the CAMCO one to allow air to escape while the water goes in the tank.
Due to the odd design of my filler tube to the tank, I made a filler adapter that reaches all the way to the tank itself. I made mine with Tygon tubing and a brass water hose adapter.
Charles