I just delt with this when I sent my tank to syko lincoln to have him paint it. its not as simple as it sounds, removing and turning upside down to empty it doesnt work completely: theres a lip around the hole that keeps some in. I ended up shaking it upside down and then stuck some shop towels in it to soak up the last few ounces worth that I couldnt remove. you could also remove the petcock completely, the stem on the petcock actually makes it hard to get every little bit out of the tank so taking it all the way out helps too.
I ended up spraying it out with brake cleaner and then forcing high pressure air through to dry it. Wrapped the tank in 2 trash bags and boxed it up, then sprayed some axe body spray to mask the smell. The usps, ups, and fed ex are anal about things smelling like gas when you ship them
but shipping an entire trike is different, good luck getting the gas out!