Cats normally only pee once or twice a day, unless they have medical issues. And they are unlikely to be willing to pee in a strange situation.
On a 12 hour drive, I stopped for dinner and left my cat alone in the rig, out of the carrier and with the cat box, but he still did not go. 6 hours, I think she would just hold it.
Consider a large carrier, and I do think it's a good idea to let her of the carrier (inside the truck or trailer), at least when you stop for breaks. 6 hours is a long time to be unable to stretch or walk around. This could also be her pee break if she wants one.
My cat rides in the van without a carrier. He has a hidden nest behind the fridge. Can she have a spot behind the truck seat? Some cats are troublesome if they are free inside the rig, but, especially with an older cat, she may just find a spot and hide, or sit on your lap.