No need to fill up the hose with water, or suck on the tube, or use a pump. A siphon will begin as soon as an unbroken water column in the hose is lower than the surface of the water in the tank.
So all you have to do is fill up the gravel vac with water then lift it with the base higher than where the hose connects. You will see that the water quickly runs into the tube. As soon and the water in the hose is lower than the top of the tank put the gravel vac back into the water with the base higher so that the air escapes and it fills with water. Then just let it drain. The force of the water falling from the tank is more than enough to push it over the edge of the bathtub.
Do the gravel vac when the tank is mostly full because the amount of suction will decrease as the level in the tank falls. The less difference in height there is from surface of the tank to the end of the hose equals a decrease force driving the water through the siphon. This is why if you have a long hose and can run in down some stairs you'll get a lot more suction.
Ahhh. I will try that next water change! Thanks