Fuel tank will not fill completely

By my count my fuel tank needed about 10 gals to fill up today (total fill up is 21 gals). It took only about 2 gallons then filled to the cap and no more would fit. The gauge would show only 1/2 full so I took off and took 7 gals of gas with me in separate containers because I just had a feeling that I would run out of gas eventhough as I said no more gas gas would go into the tank. Sure enough eventually I ran out and put the 7 gals in.
The bottom line is that the tank will only take about 1/2 of it's capacity then overfill yet it's not full based on my running out of gas and based on the gauge. Some help would be so appreciated. In case it's important; the boat is a 95 Four Winns Freedom 17' and the motor is a 96 130 HP Johnson looper. The fuel tank is a permanent plastic 21 gal tank under the floor.
George

I had one that did that. I blamed it on not being vented right and a bend in the filler pipe. After a few trys at fixing and some years of use, it was still doing it when i got rid of it.

Check your vent and make sure it is open . I used weewacker line to open mine. You coud unhook from boat and tank to check it and not push the debri in tank. I also have a basstracker that is hard to fill but will filli.f you take your time. I think it vents by at filler hole and has no vent hose. I don't know what you have but is not filling because air is trapped in tank.

I'd agree it's a tank vent problem.

The problem is that I have looked all over for a vent on the boat itself and nothing. I followed the vent hose from the tank and it seems to go up right next to the fat filler hose then I can't see it ending anywhere near the boat in that general area. It almost seems like it may wind up into the underside of the gas cap. Could that be possible?

They are often stuck back into the filler pipe.

Did you verify your sender is working? Here is a picture showing a filler with an air vent connected back to the filler cap ..

Yesterday I reached next to under the cap and it's a similar to above drawing. I followed it to the tank. I disconnected it at the tank then blew in at that end and clear all the way to the cap. Boy now I'm really confused. I guess when I filled up originally I did it too quick (from a container on my lift) and that must have filled the air hose with gas just enough to make it overfill at the cap. I'm going to try one more time to fill it and just go VERY slow this time. Thanks for help guys.

Not to change the subject but while hanging around the dock I threw a line in and caught a small fish then something janked the line and I lost hook and all. I tried again and same thing happened. Third time I pulled a large brim with scratches on the side of it (bite marks). I threw him back in with stronger line and bigger hook and a big one hit it. He fought but I pulled him on the dock. It turned out to be a 20LB snook 42" long. I took pic and threw it back and off he went.