OK, the best treatment for mange is a topical treatment called Revolution. The drug name is Selamectin. You just put a single drop on the skin of the back of the neck, just below the ears. This place sells it:
https://www.joespetmeds.com/flea-tic...-cats-dogs.htm
I would communicate with this company before you order because they are in Australia and you may run into issues with them shipping it due to the tariff situation. If so, there is another treatment you should be able to buy locally, which is equine ivermectin, which can be obtained at places like Tractor Supply - anywhere that sells items for livestock should have it. It is a bit more of a pain in the rear and we would want to work closely with you on the dosing, but it IS quite effective.
Let us know which you end up with.