Grab a regular gasoline storage can and a big funnel. Drain the gas into the can and store it out of the sun and away from heat sources, etc. Now if you're concerned about vapors, remove the gas cap and attach an air hose to the tank outlet. Run air through the tank for a few minutes and you'll purge out enough vapors to eliminate any risk while working around the tank.

I have soldered and/or welded on gas tanks by filling the empty tank with CO2 or even exhaust gas, to purge the oxygen out of the tank. No oxygen, no possibility of fire or explosion!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!