I have a 1994 F150 302 with dual tanks. Gas gauge in the front tank shows full but there are only a couple of gallons in the tank. When my truck is running, gas is spewing (like pressurized) out of the gas cap area.
Sounds to me like there’s more than a couple gallons of fuel in that tank.
I’m guessing that the fuel pressure regulator return is returning fuel to that tank even though you’ve selected the other tank. What doesn’t get used by the engine goes straight back to the now full, and overflowing, tank.
It also explains why you’re getting such rotten gas mileage, which you forgot to mention.