It's much easier if you can drain the tank.
15 gallons is a lot to contend with.
I was fortunate enough to have an oil extractor that worked fast on the 5 gallons I had in there.
If you got a cheap $10 siphon pump and 3-4 5-gallon gas cans, that would work too.
It may be possible to pinch off the rubber portion of the supply line. They make special hose-pinching tools just for this purpose, though you can use a big pair of visegrips and 2 pencils, chopsticks, or equivalent. The idea is to clamp it off without damaging the hose. Place one chopstick on either side of the hose, and squeeze them together with the visegrips.
1986 300E 5-Speed 240k mi.