Hubby and I own a hauling business, (shameless plug) H4 Services - Home
It depends on the horse's on board, but in general if they travel well he just keeps going. When he stops for fuel they get a chance to rest, he will top up water and hay bags. If it's more than 10-12 hour's he'll find a nice place to pull over for a nap in the reach of the trailer. Again, he'll top up water and hay.
We don't tie any horse unless they're a danger to themselves or other horses on board. This is also better for their air ways, they can get their heads down and sneeze.
If the owners want shipping boots, they're on only for as long as they stay up, which usually on a long trip isn't very long....LOL. We have hauled 1000's of horse's and believe me shipping boots are for you the horse could care a less, they do just fine without. If they're nut balls well maybe......we have video display in the truck so we can see the second something isn't right.
We ask all of our customers to not feed grain the day of hauling
and we only feed a local grass hay, nothing rich. We also suggest that they don't grain for a day or so after the haul if it's a long one.
Electrolytes for a day or so before is good idea, if they won't drink on the trip then giving more is fine too. One trick that never fails is pack some cheap beer with you. Empty the water bucket, pour in 1/2 a beer and let the horse drink it down. They rinse the bucket and put in fresh water, they'll drink!!!!
We also have dry hay cubes in the trailer that we can soak to add more water into them.
For your own horse's you elect to take them off the trailer, we never do but they're not our horses. Like Kevin said just the trailer not moving is a rest for them. Too many things can go wrong letting them off the trailer, they are much safer left inside.