I think my 2006 Sea Breeze has the same fuel indication problem you have.
My take is that the fuel sending unit only covers the range from empty to about 50 gallons of a 75 gallon tank. The top 25 gallon portion of the tank is above the physical range of the fuel sending unit.
My fuel gauge does not move until after about 200 miles (25 gallons) and then tracks down normally until the tank is almost empty (about 500 miles).
I have no problems filling the tank to full.