Seeing as how you work fulltime, i'm not sure if this will work but i'll say it anyway. I have a sister-in-law who is partial OCD so she is always on top of things. She has four kids and is constantly on them about using the bathroom. Her oldest is going to be 12 who is also a down-syndrome child. So like I said, she is constantly reminding them, sometimes even telling/demanding them to use the restroom. I have a 7 year old myself and I know how easy it is for them to forget or lose attention. Anyway, the best I can suggest is before you go on a car ride, have her use the restroom. As soon as you arrive where your at, use the restroom. Before you leave, use the restroom. Of course you want to make it about every one and half to two hours between bathroom visits so that they don't cause an infection because of constant bathroom usage (not sure if that is possible, but you can never be sure, this is just my opinion). Other than that, good luck with your daughter, I hope everything works out.
Another thing to keep in mind, make sure she is not doing it for attention because that could be the case. It could probably be a seperation anxiety or something of that sort.