In the past, we have gone as far as removing the inline fuel filter and using a catch basin to collect the gas. However, we did have to cut the neck of the tank in order to drop it completely one time. I do NOT recommend doing this, as your gauge will never read correctly again.
Hopefully someone else will have a better way to do this, but letting the gas drain from the fuel filter is a long, yet effective method to drain the tank. You may need to jack the rear up to get the last few gallons out.