Jamie, I would say that you know best what is the actual root of the problem is. I would definitely do some test runs before the long drive, though.
Gravol can be used in dogs, you can ask your vet about the exact dose for his size. Ace is also an option, and since he is anxious, and not actually fearful, I would say it's a plausible one. Just be cautious with the dose.
Personally, I think that if you can "reset" his mindset about the car, it might work. The right medication might enable him to realize it's groovy in the car. I'm only saying this because of the way you describe his behaviour otherwise, and because I know what a well adjusted dog Ryuk is, and how amazing you are with him.
If you can, in the future, I might recommend getting a crate in there. I know you had a bad experience in the past with it, but I've had several wacko dogs do really well crated in the car. Or, a totally different route, if he is truly not feeling well sometimes still - some dogs (like people), who are car-sick prone (me included!) need to be able to see out the front window, as the landscape whipping by the side windows makes things worse.
If it were my dog, at this point, I would try Ace, despite the poor reviews posted above, I've seen it work very well.

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Ace is also an option, and since he is anxious, and not actually fearful, I would say it's a plausible one. Just be cautious with the dose

Fear comes from anxiety. Thunder phobic dogs usually start off a bit anxious and it progresses from there with each exposure.

Drugs like valium help change the mental state instead of just sedating the dog. I've used valium for thunderstorms for about 1.5 years with one of my dogs. I've noticed with him, the valium doesn't sedate him too much, but it does takes the edge off so he can relax. He's just a more mellow version of his normal self. Instead of being underfoot staring at me anxiously, he's happy to curl up and chill. I also noticed after about a year of giving it to him for storms (maybe a total of a dozen times) if we had a storm come through in the night even without the meds he didn't wake me up with his trembling because it was shaking the bed. He'd just curl up tight against me and go back to sleep.

Anyways, that's my 2 cents and them some! I know Jamie will make the choice she feels will work best for Ryuk!! I can imagine how frustrating this is.

have you given any thought to the DAP products sold over the counter? They are made to treat anxiety and can either come in a spray or a collar. You would want to use it prior to taking the trip before the dog begins to get anxious and knows a car trip is forth coming. The product is made by Farnam companies and sold thru their veterinary division VPL. It's a canine pheramone and is a natural secretion that the mother puts out to her puppies when they are young that comforts them. I worked for Farnam when they produced the product and have first hand knowledge of the studies and efficacy of the product. They have products for cats to help them with inappropriate urination and horses for trailering and veterinary visits.

You can buy on line thru catalogs or at Petsmart type stores.

As far as Ace, smaller doses can be used to just take the edge off the dog and does not always produce the stupor and drooling described. I give it IM and have used it for years and have never had those issues with it - but also use smaller doses.

Valium is also an option, though it does have a short half-life, and I believe a high chance of a paradoxical reaction than a lot of other drugs. I know you don't have a lot of time Jamie, but maybe you can get to the vet, and do a test run with one or the other before you go?
Good luck!

Sorry if I misread your post, but have you tried sitting in the back with him - make him hold a down using your commands - and have someone else drive. You could do a marker training session while the car is moving.

Tire him out with physical exercise, then move to the car to do the mental exercise.

Once you see that he can maitain a down with you beside him, you could attach his harness to the "anchor" system of your car so that he must stay in a down position - then you can just reinforce with treats thrown to the back and your voice.

Ace is pretty safe. I'm thinking about 10 mg for a 50-60 lb dog, but I may have forgotten the dose. Try it in the evening, so he can sleep it off overnight. Ace is an antihistamine, a dramamine with a wollop, it is a phenothiazine tranquilizer - it limits secretions, quiets the stomach. The problems with it are its long duration (like 6=8 hours) and IT ELIMINATES TRAINING, so the dog has to be leashed. You can tell when the ace is on board because the 3rd eyelid will protrude.
It's cheap and works for many dogs, but some evidently do not like to be high, for those it does not work well.

I don't think Ace is a dissociative anaesthetic, it is used with a dissociative anaesthetic (ketamine). THAT one should be on the back shelf IMO, unless you are dealing with a cat or a horse, in which case, a little dis-association is often a great thing. JMO.

Do you think the harness/seat belt set up might be making him more nervous by feeling restrained? My husky acts crazy if I don't take off his sledding harness before loading him in the car. The one time I tried this, I had to pull over and remove it. He was acting like you describe Ryuk, up and panting and a little out of his mind. It took me a while to figure out what was going on, but he settled down as soon as I removed the harness. Just a thought.

Maybe a crate is the best option for safety?

One of my friends used to give her lab some benedryl when he was hyper or overwhelmed with flea allergies and she needed to get work done. This was on a vet's recommendation, but I don't know the safety or effectiveness of this treatment and have never used it on my own dogs.

Betty or a vet tech may be able to tell you about benedryl.

Yes, benadryl. I use it with my two anytime there are going to be fireworks (mostly to keep them from barking incessantly).

You can give them the same dose you would for a child of the same weight.

Meclizine also works and is often prescribed by vets for things like labrynthitis and car sickness. I believe you can give them up to 25 mg. but you might ask your vet.

My Rottie does nothing but stand in the car as well - even driving home from Idaho 3 days after having emergency surgery while we are on vacation there. I've learned to just let her stand. I think it's more stressful for me than it is for her.