Grey background or the desktop background? Do you have full screen mode enabled? If so turn it off.
If you are seeing the desktop backgound on both screens your tv connection is working and if the menu bar is on one screen and not the other you are running the screens in extended display mode.
About Arrangements, Menu Bar, Extended Displays, and Mirrored Displays
When you have multiple monitors (and the TV is considered just a monitor) only one monitor is designated as the "main" monitor. That's the one with the menu bar.
If you look at the Arrangement tab in your Displays preferences you will see both monitors. One will have a little white "menu bar" on it. That's your main monitor. You can drag the little menu bar to the other monitor if you wish to make it the main monitor.
You cannot make both monitors main monitors. However there is a third party app called MenuEverywhere that allows you to do that (it has mixed reviews).
Your two monitors can be spatially arranged with the Arrangement in Displays as well. Drag the monitors around each other to define their relationship. Then you can drag the mouse or windows across with in that "space". This is extended desktop mode and I recommend that to be able to use all the available screen real estate. However there is a checkbox in the Arrangement to run in mirrored mode. Now the two images are identical on both screens albeit the max screen size is determined by the smallest screen in the mirrored set. You would of course have the menu bar on both screens since they are mirrors of each other. But I think it's a waste of good screen real estate.