Are you sure it is emptying completely on empty?
Is it really full when the sensor says full?
Might just be some crud on the inside of the tank shorting the sensors. Add some dishwasher soap and hot water to the tank and drive a few miles on a slightly rough road then empty. Also check the wiring to the three level sensors on the outside of the tank. Do the 1/4 1/2 and 3/4 level indicators progressively illuminate as you add you 3.5 gals ?
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