Sounds like this is becoming more of a power struggle than anything else. My daughter was like that. Shes 4 now but she used the potty as a power struggle.
How do you react to her when she pees in her panties? Have there been any significant changes in your home lately?
With my daughter I had to force myself to give it up. I would get angry at her and do the whole punishment routine of taking things away and taking priveledges away but that really didnt solve anything. Then I would do the bribery/reward route and that really didnt do anything either. It was her way of getting a rise out of me and I pretty much fell into it.
What I honestly would do is just leave the issue alone and let her know that if she cant make it to the toilet, maybe she needs to wear pull-ups. Sit her down and let her know that it is not okay to pee in her panties so if she cant make it to the toilet, you'll have to take her big girl panties away and she will have to wear diapers again. I honestly would make a big deal about the panties, even buy her new ones with princesses on them (or whatever she likes), sparkly ones, whatever. And tell her that when she can start using the potty again and staying dry that she can have them. Then put her in pull-ups and leave it alone.
I finally just left the issue alone and then one day, she just decided she wanted to go all on her own. No pressure from mom. My babysitter did have alot to do with it too. She was someone my daughter really looks up to and she made it clear to her that she did not like her wearing pull-ups and that she didnt like her smelling like pee. So that was that. My daughter didnt want to hear it from me, it really took someone elses input.
But really by not stressing it anymore, I took the power away from her. She was manipulating me by peeing in her pants. I had just had a baby when all this was happening so she was losing some of my attention and using the potty as a way to get it back.
I do like the idea of keeping a potty in the car when you are out and about. We do that. I dont have the portable one. I just have a regular potty in the back of my car. I usually put a plastic shopping bag in the potty section and pad it with paper towels so if she has to go, while we are out, its easy clean up. Plus I'm not much of a public bathroom person myself. I dont blame your daughter for not wanting to use it. Most are really yucky.
Flushable wipes are excellent for making sure they clean themselves really well. We also use a squeeze bottle full of water. It helps my daughter get herself really clean while we are at home. Also make sure that her underwear is 100% cotton anything else could cause bacteria growth. Cotton is the most breathable. Also being that it is the winter time, if shes wearing alot of tights that could contribute to a smell down there. Again, not very breathable.
So I know, wordy and I tend to ramble but I hope that some of this helps. ;)