Often better to do the fades after doing a mix to stereo wave file. (or mp3, or mp4 or whatever)

Or you can position cursor at start of fade.
Open Mixer
Hit the record button on the mixer window to 'record mixer moves'
Then use the 'All fader to fade all the tracks out.
Hit Stop on main window

(you'll learn CTRL+8 keyboard shortcut to open mixer window again)
Set All fader back to full
Rewind to before the fade and hit Play
You should see all the track faders move with the same fade you recorded with the All slider.

It's weird to some, as on playback the fade happens per track (and the All slider stays put), but when you recorded it you used the All slider...
I like this solution as it allows resetting the main volume and the recording from the All slider just gets written to all the tracks.

I do not work here, but the benefits are still awesome
Make your sound your own!