Fuel gauge only works at full then drops to empty help

[Lzl 720]

I got 1984 720 and my fuel gauge work when i fully fill it up all the way but after about 15 mins it drops to empty and just doesnt work again im very confused and dont wanna buy anything that isnt nessecary too. If u have any input on  what could be causing this let me know please so i know where to start checking first thank you very much 

[]2eDeYe]

Sounds like the float is filling up with fuel or some other issue with the sending unit. 

You will need to drop the tank to check it out. 

[Buckner99]

Drop the tank or lift the bed or red neck it and cut a hole in the bed