Hello A.,
Not sure what type of vehicle you have but what I saw one mom do, in the zoo's parking lot, was to have her daughter sit on a potty chair that she had placed in the back of her SUV. I'm assuming that she probably had her sit on it before and after taking her to the zoo. I beleive you could probably use a plastic bag to go over the potty bowl and then dispose of it that way? Or I think they may even sell portable potty's for this purpose? You can try doing a google search.
Also, another thing that I recently heard about is putting a pair of the cotton training pants on the child and then putting a pull-up over it. That way if your child has an accident while you are out-and-about she is not soaked but still "feels" wet and uncomfortable thus causing her to want to avoid future accidents.
In addition to the above tips, just make sure that she uses the restromm at home right before you leave even if she says that she doesn't have to go and at the location (if one is available) and bring a change of clothing along as well.