Fuel tank / pump issue?

I bought this truck used and quickly discovered that it runs out of gas with the fuel gage showing 1/4 full.
At first, I figured the gauge was not accurate and I can live with knowing Empty is 1/4 tank. So, after putting in 2 gallons, and got off the side of the road to a gas station. I put in about 10$ worth of gas to get home. Later in the week the truck started to sputter again at a 1/4 tank. Lucky I was near a gas station. This time I filled the tank to the top and found that it would only take 15 gallons max. The owner's guide and web searches confirm that this trunk has a 22.5-gallon fuel tank capacity.
My Question is Where is the other 7.5 gallons hiding?
I now believe the fuel gage is accurate but for unknown reasons the fuel pump won't work at less than a 1/4 full.
I changed the fuel pump relay and fuel filter. Looks to me like the previous owner recently replaced the fuel filter too.

Is there anything else I can do before taking the truck in to replace the fuel pump assembly? This is an expensive repair at the local shop.