I taught my ASD DS to use the toilet and I can tell you how I did it, but it was hard work and my heart goes out to you. What I did was this: a) put him on the loo on one of those kids' loo seats every time you're running the bath, let him sit there, put posters on the wall, give him books, make the loo a "desination of choice " for your DD b) start saying wee-wee while DD on loo, and look all excited and encourage it (nb my DS was non-verbal at the time so this bit might be different for you as your DD is verbal) c) if any wee or poo ever goes in the loo, give the most massive amounts of praise immediately and also pop a choc in her mouth, so she gets the immediate reward effect d) do this again and again, every night, and leave her loo seat on so she knows it's her space
e) predict when she needs a poo or wee (I reckoned after a meal for the former and 2 hours after a drink for wee) and put her on the loo up to an hour before, with books etc. It sounds harsh, but I would keep my DS on there for a long time, till he really had to do the poo. Then chocs again.
f) one day, after a few months of this, he did his business in the loo and waved the choc away, as if to say I don't need that now mum.
The other side of the coin, which not everyone will agree with, but which I did via my ABA program, is that whenever he had an accident I took him upstairs and gave him a full bath/shower. I also got angry at him, which I know is not what the experts say, but it worked for my DS as he likes my happy face not my angry one! He found this all a pain, and it was a big pain for me, but I had to show him it's not on - for his dignity as well as my sanity.
Two years on, cross fingers, he is better than his NF sisters at using the loo (though I still do his wiping, must admit) He has also just started asking for the loo, in his sweet little word "weetoi" - which means wee on toilet!
Good luck Knat, I hope something in here is of use, though of course every child is so different.