HELP! My puppy has mange! :*(

So I rescued a 4 month old German Shepherd approx a month or two ago. I named her Lily. She's gorgeous and supa smart! She LOVES to dig in my back yard! A couple weeks ago I noticed a spot of what looked like dirt on her nose. A day or two after that I noticed a similar spot on her front paw...I then recognized that it was identical to the spot on her nose...with further investigation I realized that it was a spot of thinning hair, or dry skin, and NOT dirt. I started to get concerned that Lily had some sort of a skin disorder so off to the vet we went. After a skin scraping the vet informed me that my sweet Lily has mange. He told me that it's not contagious and it's actually caused by a weak immune system, which is apparently common in GSD's. He gave me some sort of a cream to put on the localized spots and said it should kill the mange. He said that if it doesn't get better, or if it spreads at all then I would have to take her back in for some sort of a "dip". But he didn't have any suggestions on how to boost her immune system to stop it from occurring.So...what I've been doing is using the cream and giving her a couple of drops of doTerra brand essential oil called OnGaurd (to help boost immune system) daily. I'm horrible at time frames, but I think I've been doing this for approx 2 weeks. I don't think it's getting worse, but I know it's not getting better. I'm starting to freak out a bit....I REALLY don't want to have to put her through some sort of a chemical dip..but I can tell she's not happy...she's itchy and she chews on her paw a lot. I feel bad for her and I just don't know what else to do....I have 2 other dogs and a cat and they are all fine.Anyway...any suggestions? Help? Thoughts? Anything?Thanks! :D