I hauled three of my horses, myself, from the OH/Pa border to SoCal in a 4-horse open stock bumper trailer ------------ five years later I hauled them back to Southern Middle Tennessee, where we are now retired.
I made both trips in 5 days/4 nights.
If whomever's driving the truck is a seasoned driver in all sorts of road "events" and can stay awake, the 12 hours is doable. There were times we were a few hours late for a layover due to road construction; that meant my horses were in the trailer nearly 12 hours.
The vet that gave all my horses physicals and my health certs to travel with (GET A HEALTH CERT!), hauled her horse back and forth to vet school, which was 10 -12 hours away.
She told me since my horses were not event horses in superior condition (meaning well muscled
, they would actually make the trip better than a horse in top condition.
She also said it would be way less stress on them for me to haul them as they were in their own trailer and understood how I drive. They might have each lost 20 - 30 lbs going to California but I don't think anyone lost anything coming back to Tennessee.
Do NOT let the horses out at a rest stop for anything. If you think you want to lay over or rest the horses, let them spend a night somewhere and that's when they get taken off the trailer.
I can't say enough good things about www.horsemotel.com
I only gave The Boys, literally, a handful of grain, once we stopped for the night but made sure they always had hay in front of them in the trailer. I offered them water every time I stopped for gas which was often in my ancient GMC with a big block that you have sneak up on it with the key, just to get it started.
No water out of gas station hoses. Buy purified water inside the store and offer that to your horse when you stop.
No sheets but they all wore face masks because my trailer's an open stock.
If your daughter does take her horse, just be sure it is UTD on shots, tell the vet where the horse will be living so he/she can check with vet's in that area regarding what shots are needed, and make sure Coggins and the Health Cert are current. Last I knew Health Certs need to be within 30 days of traveling.
Taking the horse isn't near as big a deal a letting your daughter go
That's a big lump-in-the-throat-moment for every mom
Hope this helps