Should an expansion tank (for a water heater) be installed in the vertical position? Is standard copper tubing strong enough to support the tank in a horizontal position?

They should be installed vertically, up or down. However, I have seen them passed horizontally with a support strap around the tank.
https://tse1.mm.bing.net/th?id=OIP.M7d0956249736ddbb30031403ab91adf1H0&w=133&h=105&c=7&rs=1&qlt=90&pid=3.1&rm=2 (https://www.bing.com/images/search?q=expansion+tank+installation&id=8B3F5920DFFBC15E21BCFBDABFA3C662BFA62BA9&FORM=IQFRBA)

Hello Skiing4free,

Please open the links below will show how to install the expansion tanks, I agree with Ma0641 to install support strap around the tank, goodluck.

John

https://www.youtube.com/watch?v=4G7QUl6FrJ0

https://www.youtube.com/watch?v=RDuMPeKx9_U