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What does the tank gauge say? /3?
More than likely, the tank gauge is faulty. Those can be replaced. I would keep an eye on the usage and see how many gallons you put in when the tank gauge shows about empty and then you can tell. You should be able to know about how many gallons it takes when empty and it is on the tank if you don't. Another way is to try to fill the tank and see if it takes any. IF NOT, you know the inside gauge is correct.
O.K........but, one of the gauges is wrong. I am trying to give you some info on how you tell which one that you can do without some electronic test equipment.
And are you sure that is for LP and not fresh water? Easy to confuse these sometimes.