I have had tanks belch fuel if parked on an incline, but it sounds like your fuel gauge isn't working and your tank is full. The filler tube is the largest diameter I have seen behind a fuel door and attendants remark about that all the time. In Oregon you can't put gas in your own vehicle, diesel you can, but most don't and some stations can get upset if you do. So I just hand them the card and hang around to make sure they reach for the diesel and not gas. I think your tank is full unless it is damaged or someone swapped in another tank that doesn't fit right. If you have fuel standing in the neck and you have an original neck and tank that doesn't have something down there blocking it, your tank is full.
Since we can't see inside the tank, sometimes we think there is something going on in there that isn't. I bought an old 1956 International that wouldn't run the owner had tried everything, new points, cap and rotor, fuel pump etc and gave up. I tried the usual and found nothing was coming from the tank. It took compressed air to blow back into the tank. I thought someone had stuffed a rag or something in the tank as a prank. I fished around in there with a coat hanger but couldn't find anything. I was sure there was something, but I couldn't catch it. I drove the truck around a little, and it stopped again. Blew out the line, more fishing...no result. Decided to pull the tank to get that **** thing. What it was, was rust getting sucked up and plugging the line.
I bought a M1028 on GSA a few years back, they said it had all these engine problems and had been parked for 3 years. It was a beautiful low mileage truck. Perfect rust free body, I got it very cheap. Towed it home. Put in new batteries, opened the fuel bleeder on the filter to make sure it was getting fuel and smelled something funny, like gas. Smelled the tank and it was full of 3 year old gas. Sucked out 20 gallons of gas and she ran beautifully.
The moral of all this is that the fuel tank while not mysterious can be the source of many problems and yours points to the fuel gauge.