Niggross
The Elvis Impersonator school most likely had a giant collection of a vast amount of video, audio and written works about Elvis Presley. Most of the stuff would have been lost by time, but not all of it. So when the group of the “Kings” settled in the place and looked through all the recorded content, they most likely would have been inspired by it all and based their new gang around the material it recorded and teached. Kind of like a cult/religious group but instead based around Evlis and with a wasteland flair. Thats my best guess without looking into the lore of it all.