Fuel Gauge or Tank on International 4300

Not sure if this is the right sub. I have a 2017 International 4300 Box Truck. When I put fuel to the point where it almost spills out of the tank, the gauge reads it as between 1/2 and 3/4 of a tank. If I go slow enough I am still able to get some fuel in there without it spilling out, but it would take me 7 business days to keep filling it up it seems like.

I thought it might be the black tube that vents the fumes coming out of the top of the tank. Maybe it was clogged? It was kind of awkward to try to take the tube off to make sure it was not obstructed so I just pushes some air through it and the tank overflowed a little. It does not seem to be a venting problem.

Before I go and get fleeced for a diagnostic fee, can someone maybe give me some insight on what this might be? Thanks in advance.