After removing tank, Start out with a couple gallons of fresh gas,
slosh it around, and pour off for later use. Stale / stabilized gas
evaporates * very slowly * Fresh gas evaporates fast.
Place tank in sunny location, remove cap, hook a long hose to
fuel feed, pump Nitrogen or Helium into tank to displace fumes, do a
little at a time. Compressed air will oxygenate the fuel, and turn the
tank into a bomb. When liquid is removed, oil up sender unit, apply
shaving cream to bolts [ eliminates spark hazard ] Once sender is out,
air can enter baffled areas. and tank would be much safer. Replace
sender, tape up holes, and give a second blast of inert gas.
A Nitrogen outfit, Small bottle, cheap torch [ on / off valve + hose
] Regulator could be bought A CO2 outfit would be cheaper, but I
don't like CO2 for purging, trapped moisture not found in inert gas
Failing that, a party balloon kit [ Helium ] can be had cheaply,
you get a disposable cylinder of Helium, hose and balloons
Why not take tank to radiator shop, where they just dunk it
in hot, acid treated water, prior to soldering? Don't use your
garden hose on tank, environmentally uncool !