Sounds like your doing everything right, I've delt with ferals and semi
feral cats for years, and I've learned it just takes time. Once they get
to trust you, they get better, but they will ONLY trust you. Or people
in your home that give them simlar attention, so when you do take her to
the vet, don't be surprised when she becomes a wild cat for your vet.
Don't worry about it though, it's not like the vet hasn't seen this
before.

Hang in there, they are worth all the time it takes to totally tame them
And God Bless you for taking the time to give a home to the sweetie.

Aimee