Training a foster dog abused for poor housetraining

Question
I have a 7-year old spayed Papillon who was abused as my foster dog.  Apparently, some of the abuse came from housetraining problems.  The first few days I had her, she was holding her urine for entire days while she tried to find a secret place to go.  We can go out to the yard with the other dogs, and she will not go.  We can walk for long walks, and she will not go.  I have NEVER seen her go in the two weeks I've had her.  But she does, I smell it.  I find her poo of course too.  I've quietly picked up her accidents without any word to her while I tried to earn her trust.  She is better with the trust issue now.  I do not know how to even train her if she won't allow me to see her go.  She is food motivated, but I can't reward her.  I thought of rewarding my two dogs when they go so she can see what a reward will get you.  But I'm not sure if she will just lose trust or make the connection.  I'm completely perplexed about this dog, and she'll never get adopted with these kinds of problems.  I haven't tried crating her because I think she will just hold it forever, and then sneak to go when she gets out.  But I can try it now.  I appreciate any advice you can give.

Answer
HI Gale,

This is a tough one for sure: 7 years old and taught that peeing in front of people is bad.  You'd be surprised at how many people actually and inadvertently train that into dogs, thinking they are training the dog not to pee in the house but in reality the dog learns peeing in secret (in the house) is safe but peeing in front of people is unsafe. This girl still needs to be crate trained if she likes her crate and won't soil it. When she is out of the crate she goes outside. If she will not eliminate, she either goes into the crate or is tethered to you so she can't wander away and eliminate in secret in the house. If you are 'finding' it she has too much freedom in the house; she needs to be treated as if she's a puppy. If she finally does squat to pee in front of you, the best thing is to let her and reward her. If you scoop her up to take her out at this point, she will lose the trust you've worked so hard to build. You need to teach her it's ok and safe to pee in front of you and then you can progress to peeing in the correct spot. You could reward your other dogs for peeing outside so she sees them getting treats but I'm not sure she'll make the connection that if SHE pees outside, SHE'LL get a treat. Dogs aren't great mimics although they certainly can be with some behaviors.
Now that the weather is warming some, maybe you could spend more time outside with her and catch her peeing if she 'thinks' you aren't watching her? If you are familiar with clicker training, perhaps you could watch from an open window and click her peeing outside but then of course you'd need a way to get the food to her quickly.  And before you did that you'd have to teach her that the click means food 100% of the time.

Jaz