Update. I had a very helpful Slingbox support rep who was able to actually test the iPhone app with a Roku 2, Roku3 and Apple TV all online at the same time. She had no problem seeing all three. The Apple TV was avaiable through the AirPlay button and the Roku 2 and Roku both showed up via the Roku button.
The Apple TV has been unplugged for a few days, but the only was the Roku button showed up was to shut down Slingplayer, reboot the Roku, start Roku and then start Slingplayer.
Has anyone had issues like this? Is there a successful order of powering on that stays over a number of days?