emptying my fuel tank?

If its mainly petrol in the mix be very cautious.
Garages use a dedicated vacuum pump into a safe sealed container.
Your big problem is HIGHLY INFLAMMABLE fumes or vapours escaping form the container as it fills since its mostly petrol so far riskier than diesel fuel

If your vehicle model has an electric fuel pump you can disconnect the feed to the fuel filter and pump into a container
It is easy to cobble a manual pump (DIY STORES) to a small diameter hose which you can feed into the tank through the filler neck. Once the fuel is flowing it will syphon to a low container anyway.
If you do try it, be sure to do it in open air away from
other interferences!!!
Use a narrow neck storage container and a breezy day would help

Asanewt
January 2009