You might want to give her either Rescue Remedy or cocculine which are both herbs and thus safe for cats. Prescription medications like diazepam can make a kitty too sleepy and she can smother in her blanket or pillow in her carrier. Talk to her all as Hissy says - when she is calm and tranquille -try to make it as normal as possible and do not sound stressed yourself!
I travel with my Siamese often and she has adjusted to it since she was a baby (well 12 weeks). I do not give her cocculine any more and she thinks her carrier is sort of a little place to rest now and then. If you have not had your cat in the carrier before, place her inside and go for short drives before the long journey (that is if you have not travelled before with it, like to the vet?)
As Hissy suggests, NEVER let her out. If your carrier is large enough, place a small litter box in it. If not, do not feed her much before leaving - and feed her when you stop at night. (This is not a one way no stop trip is it?) I have a friend who did not place her cat in a carrier - the kitty was a rescue and not a people cat at all, she was not a cuddler and loved on her own terms (of course that part if pretty typical of any feline,lol). Alas, she had an accident - nothing major - but it spooked the cat enough that she bolted. This was near her parents' home (maybe an hour away - 50 miles) but C (my friend) lived four hrs away). She printed flyers and placed them everywhere near where the accident occured and investigated - as did her parents -. C is a reporter and so her work fortunately brought her past the place of the incident often. NOTHING happened for over six months.
Finally, one day she received a call from someone who said she'd been at a local store where one of the flyers was and she "pretty sure" one of her neighbours was feeding a stray cat that resembled C's cat. As she wrote in an article about the incident, it could not be hers "After all, I had last seen my cat more than a year ago". However, fate smiled on her and it WAS.
My point is and I am sorry to go on at length that please keep her in the carrier at all times. Even the best behaved and sweet cat will bolt when scared and freaked.
Good luck! Cats hate change and they are often best left in their own usual surroundings but if you absolutely have no choice, following Hissy's and these idea hopefully will make the trip better for both of you. Good luck!