Assuming you cannot drop the gas tank to gain access to the gauge, your best bet would be to employ bulk solvency inside the tank.  For this, you would run the gas down as far as practical and then add 1-2 gallons' worth of B-12 Chemtool Fuel Injector Cleaner (part #s 0101 & 0116).  If you can rock the truck to increase the contact area of the chemical with the interior of the thank, that will increase effectiveness.  Doing this on a warm day and soaking overnight would both also be helpful.  You might need to add a little gasoline to extra volume.  You're planning to run out the contents of the tank after doing the soak, just top it off with fresh gas.