Packing is like a chess game with the weather and the logistics of winterizing the property and packing... timing is everything.
One of the big stresses I have found for those folks who cover long hauls with their horses is a lack of preparation.
Sometimes not "having" the thing you need while traveling with horses, or the stress of how well a horse will haul, or concern about towing a trailer, whatever the case is, everyone, can always prepare better to decrease and diffuse the stress levels in the horse and themselves by building up to the actual haul by addressing each aspect involved in increments...
That being said I've been hauling this particular 1,400-mile (one way) route for the past 16 years. And though this trip only requires 5 turns (I'm not kidding) bringing me from the Candian border to the Mexican one- there are massive mountain climbs, stretches of complete isolation, two of the US's largest cities to pass through and the temperatures can dramatically vary with the starting location in the north literally half of what the destination in the southwest current thermometers are showing.
Though there are many aspects that I could address- here are my top 5 suggestions:
LEARN how to drive your rig.
I can't tell you the amount of stress alleviated by those who are comfortable driving because they have put in enough "practice" hours to feel confident to navigate traffic, tight turns and backing up.
Go use the neighbor's field, practice in an empty parking lot, use a competition facility. Have someone with you that will NOT (i.e. probably not your spouse/partner/or good-intentioned-know-it-all-horse-friend)stress you out.
Know your route- and alternative options- regarding refueling, overnight layovers (for horse and humans) and weather challenges.
I know Google can seem to be a godsend, but if you're anywhere remote, so often there is no coverage and you really don't want to be sorting out on your phone while driving. You also need to know your options in case weather becomes a factor (example, as I drive through one 300 mile stretch in Montana if there's a bad blizzard, the highway is literally shut down, and there is NO other similar route without a six-hour detour.)
Google isn't always accurate. Yup. It's true. So I suggest to folks always have an "old school" back-up (but current) map in hand, so that you can double-check, especially if having to make unexpected route changes.
Learn how fast you go through fuel when hauling the trailer so you can learn to plan where and when you'll need to refuel. Yes, we've all done it at least once in our lives, but to be stuck on the side of the road, waiting for road services to bring you gas can put a real damper on the trip. Plus, if you're in remote locations, there may NOT be options at every town to refuel or stations may close early depending on the day/time
Practice Loading and Unloading your Horse-
I remember years and years ago, the stress of hauling horses that had issues with loading, and always wondering if something happened, and I had to unload, would I be able to get them loaded again with ease. Many people experience this. Not fun to carry the mental stress and safety-wise, if you had to unload and reload, you need a horse that feels confident in the trailer, irrelevant of the circumstance.
BYOB- Bring your own bale...and anything else helpful.
Rest stops and Layovers... Are for RESTING
The horse is walking the entire time the trailer is moving. Whether you stop for gas, for a rest or layover, leave your horse alone. I can't tell you how many times I've watched folks "stretch" their tired horse by moving and moving them around. Let them be still.
And finally here are my personal TOP must-haves in an easy-to-access place:
Knife- whether for hay or anything else
Baling twine- it can seriously secure ANYTHING
Duct tape- another "cure-all" for the unexpected
Equine medicines & Emergency kit
Spare Halter and Lead Rope
Cash- you never know when the kind citizen helps you out in an unexpected scenario or the auto shop does a "closed hours deal" to help get you on the road faster.
Healthy Snacks and Water for you
Great music playlist or some interesting book on tape
Change of clothes, with layers for unexpected weather
Headlamp - with charged batteries
Roadside Assistance membership
Phone Car Charger
There is, of course, a lot more I could add, but this will give you a good jump start... I'm going to go put on my long johns, wool sweater, wild rag, overalls, vest, and duster and keep packing...