Need a loving home!

I posted a photo of Anna Leigh a few days ago. She adopted us about a year and a half ago. She is loving and while I've not been able to get her to use a litter box, she will go outdoors for the bathroom and after spending time outdoors comes back in. She has never had a potty accident  

I've tried several shelters to no avail. If anyone can help it would be greatly appreciated. Thank you.