Many vehicles have a filler tube with an anti-siphon feature in it, so you just take the filler tube off at the tank and put the sipon hose in at the tank nipple.It can actually be hard to drain fuel out of a tank. Aside from the restrictor flap nozzle, I don't think you can snake a siphon down into the filler neck without running into something; i could be wrong, but I've read that...??? Others????
How much old gas is there? I have a few gas cans so what I would do is siphon out all the gas you have cans to hold, then if there's a lot left in the tank, dispose of that gas and have the cans empty to drain the tank more, then put a gallon of fresh fuel in, then drive it straight to a gas station, fill it up, and put some Techron fuel system cleaner in too.
No point to adding fuel stabilizer AFTER it has sat 18 mos.