Is there a way to completely deflate an inner tube?
According to Yehuda Moon (google it), you can take a properly-sized ball bearing (or even a pebble) and put it inside the valve cap, then screw the cap halfway so that the ball/pebble will keep the valve open.

Then, you roll the tube like toothpaste, and when you finish you remove the cap and the pebble to avoid the natural elasticity of the tube to suck back some air.

Hope this helps