It’s great you are meeting with your vet!  I would ask them to give you metacam and some sub-q fluids in case you need them, and definitely bring critical care (or another recovery food) in case you need to syringe feed. You vet will be able to advise you on how to give fluids and when you should give meds or syringe feed.

I drove cross country with my rabbits when I moved and I tried to take long breaks every 4-6 hours so the buns could eat something. They didn’t always eat a lot, but I made sure they at least got something in their system. I had some salads of their favorite veggies that I would offer on breaks (rinsed so they would also get hydration), and sometimes some recovery food with a little banana mashed in to encourage them to eat it.

On my trip, one of my bunnies wouldn’t eat in the evening after about day 2, so I had to syringe feed her morning and night, and give fluids every night. She was just fine once we got to our destination though!

I set up the carriers so they had a pee pad, some litter, and a layer of hay in them. I had hard sided carriers so I had little water crocks attached to the side about half full so they didn’t splash. I’ve also heard of people using those expandable carriers so the bun can hop around a bit while they were waiting in the airport.

Be sure to bring enough hay and pellets with you to get you through any unexpected delays, especially since you have a few different modes of transportation.