I usually just pack mine in with sleeping bags (like Patricia suggested), and it drives just fine for hours! I do like to make sure the sun isn’t hitting it directly in the same spot the entire time, though.
I think loosening the strings applies to air travel, when the pressure & humidity will fluctuate quite a bit during the flight. If you’re staying in the same climate zone while driving, you should be fine!
(Just keep the car comfortable for you, and it’ll be comfortable for the harp.)
Have fun with your gig! : )