First off, Don't feel bad, I have the 20 gallon tank and when reading empty it only takes 17 gallons. I guess that's better running out of gas while the needle still shows gas. These gas gauges are not very accurate.
Second, filling them is a pain, mine works best with the gas pump handle pointing to the 5 o'clock position. I removed my drop down license plate and put it on my rear receiver, I hated the spring plate.