Campbell died on Aug. 8 in Nashville after a long battle with Alzheimer's disease. He was 81 years old.
In her interview with ABC, 71-year-old Parton said the two became friends after realizing how similar their backgrounds were.
“Glen and I were always talking about our similar backgrounds because we’re both from very large families, very musical families, and we both [were] very poor,” Parton said.
She also admired him for being open about his battle with Alzheimer's disease.
“I thought it was amazing that he opened up his life to the cameras in I’ll Be Me,” Parton said, referring to the Oscar-nominated 2014 documentary.
"When you think about great singers, you’ll think of Glen Campbell. When you think about good looks, you’ll think of Glen Campbell," Parton added. “When you think about an all-around entertainer and wonderful human being, you will think of Glen Campbell.”
Alice Cooper, who had a surprising friendship with Campbell, told ABC News that he personally saw Campbell "slipping," but there was still flashes of his humor.
“I saw him slipping," Cooper said. "You know, here’s this guy that’s sharp as a tack. He is always funny, is always on. And he would tell me the same joke. And then 20 minutes later, he’d tell me the joke again. And I started going, ‘Something’s going on.' "
“He had a crazy sense of humor, and we used to love to tell jokes,” Parton said of Campbell. “He’d laugh as hard at his own jokes as anybody did."