Fuel tank issue

I have a 21 sv.

I’m not to sure what to think of this but I have some sort of oddity when I fill up. My tank can be on empty, <1/8 of a tank and 0miles till empty and when I fill up I get maybe 21 gallons max. It’s my understanding that I have a 26 gallon fuel tank so where is this 5 gallons??? I get just over 300miles on 3/4 of a tank. At approx 1/4 left I get about 18 gallons when fill up.

All incite is appreciated.