Don't worry, most people think that, and with good reason since that is reported in many official sources. Bill Moyers, in Empire of Dreams, seems to be giving the impression that Campbell actually mentored Lucas through the screenplay in real life, which is preposterous. But that's part of the whole mythology-of-the-mythology. The Lucas-Campbell connection is way overstated, but both of them sort of went along with it, and I can kind of see why, because it was an opportunity to get young people interested in comparitive mythology. I never would have been reading Campbell at age 17 if it wasn't for Star Wars--although to be honest, I never finished Hero with a Thousand Faces. It was just excruciating, and I wanted to like it. I've picked it up every few years since, hoping my immature brain didn't appreciate it, but nope, same reaction.
As a speaker, he is phenomenal. I have a collection of bootleg lectures from him, plus a few audio cassette series he put out, and they're all fantastic. As a lecturer he was very powerful, and really did have an encyclopedia-like ability to just recall any sort of obscure story when it was relevant and connect it to his narrative. The best book with his name with it is the Power of Myth book, because it's just a transcription of an extended interview with him by Moyers.