I am not aware of a keyboard shortcut for Fade In or Fade Out and I'm not aware of any way to make one.
The right-click menu is the shortest shortcut I know.
I am assuming that you are referring to keyboard shortcuts for the fades rather than going (for video) to Applied Effects Tab/Applied Effects Palette/Opacity Panel expanded.
In my way of thinking, although not keyboard shortcuts, the Fades at the Timeline levels are the shortcuts for those effects.
I too like SG am not aware of keyboard shortcuts for these Timeline level shortcuts.
However, in case the situation ever arises, I will share with you an Opacity "keyboard shortcut" that I came across. It seems to related only to Opacity of text or embedded image in the title being created in the Titler.
In Premiere Elements 12, go to Edit Menu/Keyboard Customization/Applications and the category Text/Transform/Opacity and assign a keyboard shortcut to it, for example Shift+X. When you find yourself in the opened Titler, you can use Shift+X to adjust the Opacity of the text and/or embedded image in the title via the pop up
You still have work to do typing in the wanted Opacity.
If interested, please check out the blog article of Premiere Elements Keyboard Customization opportunities with the complete listings for Premiere Elements 11 (same as for 12). You should be able to access this list from your Premiere Elements 12 Expert workspace Edit Menu/Keyboard Customization.