I'm about to just bite the bullet with DD2 and pack plenty of clothes. With DD1 I actually put 1 of the potties in the boot. I would pull over, undo her & she would go to the potty in the boot (walk through 4wd).

Am thinking if doing that with DD2, I don't ask her anymore, if she wets herself I explain this is why we use the toilet, change her and move on.