Good idea but flow rate may be a killer.
I use a Jiggler Siphon (19mm will take about 2 minutes to do 20 litres).
https://www.google.com.au/search?q=j...w=1178&bih=912
Great tip I saw somewhere is if you use it for drums/jerries as opposed to vehicle tanks with bends in the fillers.
Get a bit of semi-rigid hose (like retic) and put it between the valve and the flexible hose.
That allows you to easily guide the valve to the lowest corner of the drum.